Synonym for error. Used for instances where the program has
decided to exit because of invalid user input, or the user pressed
quit etc. This function allows error to be reserved for
programmer errors.
The error used in fixpointToFinal, runFixpoint and
runFixpointM when the result of a failed computation is
recursively used and somehow visible. You may use this for your own
Fixpoint interpreters. The argument should be the name of the
interpreter.