Fix MachOObjectFile::getSymbolName() 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 "bad string index" for bad string indexes.  Updated the error message
in the llvm-objdump test, and added tests to show llvm-nm prints
"bad string index" and a test to print the actual bad string index value
which in this case is 0xfe000002 when printing the fields as raw hex.

llvm-svn: 258520