Improve error info in assertEmpty and assertNotEmpty

When assertEmpty fails, it will print:
        raise mismatch_error
    MismatchError: 0 != 5

this is to improve the error info as:
        raise mismatch_error
    MismatchError: 0 != 1: list is not empty: [u'1b95a8ea7e124a76bfadf6ce1906aa27']

and when assertNotEmpty fails, it will print:
        raise self.failureException(msg)
    AssertionError: 0 not greater than 0

this is to improve the error info as:
        raise self.failureException(msg)
    AssertionError: list is empty.

Change-Id: Icdf69b596fa135387721c49df164a68799ab9a70
1 file changed