Fix #92 (inappropriate error message)

[inariksit]

Fixes #92

Used to print out a suggestion "maybe you're pattern matching at runtime" always when there was an internal error in GeneratePMCFG.
Now checks what the actual error message is, and only prints out the suggestion for one specific error message.

Tested with the following grammars:

• https://gist.github.com/inariksit/88281c9698fd45a5f2bd00aaf14fa183 Example from Improvements in common error messages #55 when the inappropriate message was introduced
• The minimal example from Confusing error message: internal error in GeneratePMCFG #49 (a previous issue that led to PR Improvements in common error messages #55)
• The minimal example from Error message referring to pattern matching on runtime strings when using Prelude.notYet #92 (issue that noticed the current bug).