llvm-dwarfdump: Return non-zero on error

Makes it easier to test "this doesn't produce an error" (& indeed makes
that the implied default so we don't accidentally write tests that have
silent/sneaky errors as well as the positive behavior we're testing for)

Though the support for applying relocations is patchy enough that a
bunch of tests treat lack of relocation application as more of a warning
than an error - so rather than me trying to figure out how to add
support for a bunch of relocation types, let's degrade that to a warning
to match the usage (& indeed, it's sort of more of a tool warning anyway
- it's not that the DWARF is wrong, just that the tool can't fully cope
with it - and it's not like the tool won't dump the DWARF, it just won't
follow/render certain relocations - I guess in the most general case it
might try to render an unrelocated value & instead render something
bogus... but mostly seems to be about interesting relocations used in
eh_frame (& honestly it might be nice if we were lazier about doing this
relocation resolution anyway - if you're not dumping eh_frame, should we
really be erroring about the relocations in it?))