[llvm-nm] Fix crash when running with --print-armap on corrupt archives.

error() in llvm-nm intentionally does not return so that the callee can move on to future files/slices. When printing the archive map, this is not currently handled (the caller assumes that error() returns), so processing continues despite there being an error.

Also, change one return to a break, so that symbols can be printed even if the archive map is corrupt.

llvm-svn: 344268