Is the object a 'miraiInterrupt'. When a mirai is sent a user interrupt,
e.g. by ctrl+c during an ongoing
call_mirai, the mirai
will resolve to an empty character string classed as 'miraiInterrupt' and
'errorValue'. To test for all error conditions, including timeouts etc.,
is_error_value should be used instead.
Logical TRUE if 'x' is of class 'miraiInterrupt', FALSE otherwise.