Fix MachOObjectFile::getSymbolSection() to not call report_fatal_error()

but to return object_error::parse_failed.  Then made the code in llvm-nm
do for Mach-O files what is done in the darwin native tools which is to
print "(?,?)" or just "s" for bad section indexes.  Also added a test to show
it prints the bad section index of "42" when printing the fields as raw hex.

llvm-svn: 258434