Get the name of an opcode, given the 'op' byte sequence of the operation.

def op_get_opcode_name(op_string):
    """
    Get the name of an opcode, given the 'op' byte sequence of the operation.
    """
    global OPCODE_NAMES

    # special case...
    if op_string == '{}:'.format(NAME_REGISTRATION):
        return 'NAME_RENEWAL'

    op = op_string[0]
    if op not in OPCODE_NAMES:
        raise Exception('No such operation "{}"'.format(op))

    return OPCODE_NAMES[op]