Replace temporary error with separate error type

[wenkokke]

The following values...

temporaryUnsupportedAlternatingQuantifiers
temporaryUnsupportedNonLinearConstraint

...should be replaced with different constructors without any fields, such as...

  ...
  | BacktrackFromUnsupportedAlternatingQuantifiers
  | BacktrackFromUnsupportedNonLinearConstraint

Perhaps these constructors should even be in a different error type, so that we can statically check if we're always catching them.

[MatthewDaggitt]

Yup agreed. It's just difficult to get the monads to align, as we need to throw both the permanent errors and the temporary errors, not to mention we have a lot of helper error functions that assume we're in the permanent error monad.