Reapply: [llvm-size] Tidy up error messages (PR42970)

Clean up some formatting inconsistencies in the error messages and correctly exit with non-zero in all error cases.

Originally submitted as r374771 and then reverted as r374780, this patch fixes the libObject test case in Object/macho-invalid.test.

Patch by Alex Cameron

Differential Revision: https://reviews.llvm.org/D68906

llvm-svn: 374793