Fix several errors:
- Indented markup is now reparsed right
- All end group errors will now fail a reparse
- Rightmost errors will always fail a reparse
- Makes errors fatal, so that a phase is only reached when all previous phases were error-free
- Parsing still recovers and can produce multiple errors
- Evaluation fails fast and can thus produce only a single error (except for parse errors due to an import)
- The single error that could occur during execution is removed for now
- Removes Value::Error variant
