Report an error when dpkg-source fails to extract package.

While invoking dpkg-source to extract package, switch from os.system to Popen to get output and exit code.

A fake PluginResult is generated if extract fails.

The importer has been modified to report dpkg-source output if detected.

This has been tested with a faulty package (orig not matching):

15:17:40,070 DEBUG [debexpo.lib.plugins] Running plugins of type: qa                                                                                                                                              
15:17:40,070 DEBUG [debexpo.lib.plugins] Getting plugins with key (repr): 'debexpo.plugins.qa'                                                                                                                    
15:17:40,070 DEBUG [debexpo.lib.plugins] Using these plugins: lintian native maintaineremail watchfile closedbugs controlfields diffclean buildsystem debianqa distribution                                       
15:17:40,070 DEBUG [debexpo.lib.plugins] Extracting package for plugins                                                                                                                                           
15:17:40,070 DEBUG [debexpo.lib.plugins] Copying files to a temp directory to run dpkg-source -x on the dsc file                                                                                                  
15:17:40,073 DEBUG [debexpo.lib.plugins] Temp dir is: /tmp/tmp2vTTZJ                                                                                                                                              
15:17:40,073 DEBUG [debexpo.lib.plugins] Copying: ifmetric_0.3-1.dsc                                                                                                                                              
15:17:40,073 DEBUG [debexpo.lib.plugins] Copying: ifmetric_0.3-1.debian.tar.xz                                                                                                                                    
15:17:40,076 DEBUG [debexpo.lib.plugins] Extracting sources for ifmetric                                                                                                                                          
15:17:40,355 CRITI [debexpo.lib.plugins] Failed to extract sources for ifmetric:                                                                                                                                  
gpgv: Signature made Sat 12 Jan 2019 02:50:24 PM CET                                                                                                                                                              
gpgv:                using EDDSA key 2DE24895313C5DA7F089E14E17494B10795DD733                                                                                                                                     
gpgv:                issuer "lyknode@cilg.org"                                                                                                                                                                    
gpgv: Can't check signature: No public key                                                                                                                                                                        
dpkg-source: warning: failed to verify signature on ./ifmetric_0.3-1.dsc                                                                                                                                          
dpkg-source: error: file ./ifmetric_0.3.orig.tar.gz has size 313587 instead of expected 99874                                                                                                                     
                                                                                                                   
15:17:40,356 ERROR [debexpo.importer.7573] Rejected: QA plugins failed the package      
                                                                                                             
gpgv: Signature made Sat 12 Jan 2019 02:50:24 PM CET                                                     
gpgv:                using EDDSA key 2DE24895313C5DA7F089E14E17494B10795DD733       
gpgv:                issuer "lyknode@cilg.org"                                                          
gpgv: Can't check signature: No public key                                                            
dpkg-source: warning: failed to verify signature on ./ifmetric_0.3-1.dsc                        
dpkg-source: error: file ./ifmetric_0.3.orig.tar.gz has size 313587 instead of expected 99874                                                                                                                    
                                                                                                                                                                                                                 
15:17:40,356 DEBUG [debexpo.lib.email] Getting mail template: importer_reject_maintainer                                                                                                                         
15:17:40,373 DEBUG [debexpo.lib.email] Starting SMTP session to localhost                    
15:17:40,376 DEBUG [debexpo.lib.email] Sending email to lyknode@cilg.org                                  
15:17:40,391 DEBUG [debexpo.lib.email] Successfully sent

Closes #43 (closed)