Fix regression in JSS error handling from c8eb7a
Fix regression in JSS error handling from c8eb7a

Errors raised by `fail` were being passed up as dynamic exceptions
rather than being caught by the ExceptT handler, when changing from
ErrorT.
GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script