Clean up the specific error message for a malformed Mach-O files with bad segment

load commands.

The existing test case in test/Object/macho-invalid.test for
macho-invalid-too-small-segment-load-command has a cmdsize of 55, while
being too small also it is not a multiple of 4.  So when that check is added
this test case will produce a different error. So I constructed a new test case
that will trigger the intended error.

I also changed the error message to be consistent with the other malformed Mach-O
file error messages which prints the load command index.  I also removed both
object_error::macho_load_segment_too_small and
object_error::macho_load_segment_too_many_sections from Object/Error.h
as they are not needed and can just use object_error::parse_failed and let the
error message string distinguish the specific error.

llvm-svn: 268652