Please include stdout error messages from readelf in output

This bug was originally reported by Mike Hommey (mh+reportbug@glandium.org) in Debian bug #931963:

[..]
First, there's the problem that diffoscope only displays readelf's
stdout, and its stderr is lost, which it contains the most important
information:
  section '.eh_frame' has the NOBITS type - its contents are unreliable.
[..]