The type checker needs to look up the VTs anyway, so it makes more sense
to put the error check there as well.

This is possible now that the exceptions are unchecked.

Replaces the previous ErrorType enum. Separate classes are more
convenient for catch blocks and such.

Making the exceptions unchecked will allow throwing them from places
where checked exceptions cannot be used, e. g. SableCC AnalysisAdapter
methods.

For some reason our Windows and macOS runners don't work for this
project right now. The OS difference probably isn't relevant here
anyway.

Pulled out from probparsers repo as of commit
0942cab5e8ae41906cc33de01c5ffbd949d43a68.