Produce another specific error message for a malformed Mach-O file when a load

Produce another specific error message for a malformed Mach-O file when a load
command has a size less than 8 bytes.

I think the existing test case in test/Object/macho-invalid.test for
macho64-invalid-too-small-load-command was trying to test for this but that
test case triggered a different error given how it was constructed.  So I
constructed a new test case that would trigger this specific error.

I also changed the error message to be consistent with the other malformed Mach-O
file error messages.  I also removed object_error::macho_small_load_command from
Object/Error.h as it is not needed and can just use object_error::parse_failed
and let the error message string distinguish the error.

llvm-svn: 268463