Add more error checking to llvm-objdump and MachODump

[Rafael Espíndola via llvm-commits]

> For me it is extremely common to see broken .o files and that is what I do for a living.  And I write these tools for me to do my job.  If others find them useful great!  But they are needed to look at good and bad .o files.
>

I see files like that for the same reason. But it is important to
stress that we are not the common case. We should not make objdump
harder to maintain because a handful of developers sometimes find it
useful to see more than one error printed on a file they just created.

And given the option of multiple errors with a good chance of only the
first one being relevant or a single error that is far easier to put a
breakpoint on I would always take the single error.

Cheers,
Rafael