Catch failures to disassemble and rescue all other differences. (reproducible-builds/diffoscope!29)

When trying and failing to disassemble an ELF library from an architecture not
supported by objdump, catch that particular failure and don't let it revert the
*entire* diff to hexdump, throwing the entire kindergarten class with the
bathwater. Fallback on hexdump _only_ for the problematic text section and
rescue all the rest. Example:

Before:

diffoscope build?/app/libapp.a
--- build1/app/libapp.a
+++ build2/app/libapp.a
│┄ Command `objdump --line-numbers --disassemble --demangle --reloc
│┄  --section=.text build1/app/libapp.a` exited with return code 1. Output:
│┄     objdump: can't disassemble for architecture UNKNOWN!
│┄
│┄     objdump: section '.text' mentioned in a -j option, but not found
│┄     in any input file
│┄
@@ -1,22 +1,22 @@
 00000000: 213c 6172 6368 3e0a 2f20 2020 2020 2020  !<arch>./
 00000010: 2020 2020 2020 2020 3135 3633 3333 3638          15633368
-00000020: 3830 2020 3020 2020 2020 3020 2020 2020  80  0     0
+00000020: 3931 2020 3020 2020 2020 3020 2020 2020  91  0     0
 00000030: 3020 2020 2020 2020 3134 2020 2020 2020  0       14
 00000040: 2020 600a 0000 0001 0000 0052 6d61 696e    `........Rmain
 00000050: 0000 6d61 696e 2e63 2e6f 626a 2f20 2020  ..main.c.obj/
 00000060: 2020 3020 2020 2020 2020 2020 2020 3020    0           0
 00000070: 2020 2020 3020 2020 2020 3634 3420 2020      0     644
 00000080: 2020 3130 3332 2020 2020 2020 600a 7f45    1032      `..E
 00000090: 4c46 0101 0100 0000 0000 0000 0000 0100  LF..............
 000000a0: 2800 0100 0000 0000 0000 0000 0000 2802  (.............(.
 000000b0: 0000 0000 0005 3400 0000 0000 2800 0c00  ......4.....(...
 000000c0: 0b00 0149 0248 fff7 febf 0000 0000 0b00  ...I.H..........
 000000d0: 0000 6d70 7332 5f61 6e33 3835 0048 656c  ..mps2_an385.Hel
-000000e0: 6c6f 2057 6f72 6c64 2120 2573 0a00 0047  lo World! %s...G
+000000e0: 6c6f 2058 6f72 6c64 2120 2573 0a00 0047  lo Xorld! %s...G
 000000f0: 4343 3a20 2863 726f 7373 746f 6f6c 2d4e  CC: (crosstool-N
 00000100: 4720 312e 3234 2e30 2d72 6332 2d64 6972  G 1.24.0-rc2-dir
 00000110: 7479 2920 382e 332e 3000 412c 0000 0061  ty) 8.3.0.A,...a
 00000120: 6561 6269 0001 2200 0000 0537 2d4d 0006  eabi.."....7-M..
 00000130: 0a07 4d09 0212 0414 0115 0117 0318 0119  ..M.............
 00000140: 011a 011e 0422 0100 0000 0000 0000 0000  ....."..........
 00000150: 0000 0000 0000 0000 0000 0100 0000 0000  ................

- After:

 2019-07-17 04:15:40 E: diffoscope.comparators.elf:
 Command '['objdump', '--line-numbers', '--disassemble', '--demangle',
 '--reloc', '--section=.text.main', 'build1/app/libapp.a']' returned
 non-zero exit status 1.

2019-07-17 04:15:40 E: diffoscope.comparators.elf:
  Command '['objdump', '--disassemble', '--demangle',
  '--section=.text.main', 'build1/app/libapp.a']' returned non-zero exit
  status 1.

--- build1/app/libapp.a
+++ build2/app/libapp.a
├── file list
│ @@ -1,2 +1,2 @@
│ -----------   0        0        0       14 2019-07-17 04:14:40.000000 /
│ +----------   0        0        0       14 2019-07-17 04:14:51.000000 /
│  ?rw-r--r--   0        0        0     1032 1970-01-01 00:00:00.000000 main.c.obj
├── readelf --wide --decompress --string-dump=.rodata.main.str1.1 {}
│ @@ -1,7 +1,7 @@
│
│  File: lib.a(main.c.obj)
│
│  String dump of section '.rodata.main.str1.1':
│    [     0]  mps2_an385
│ -  [     b]  Hello World! %s^J
│ +  [     b]  Hello Xorld! %s^J

Signed-off-by: Marc Herbert <marc.herbert@gmail.com>
Signed-off-by: Chris Lamb <lamby@debian.org>