TISTUD-6683 Gracefully handle invalid XML attribute values

- be able to handle unquoted attribute values that begin with digits in our scanner
- mark unquoted attribute values as parse errors and recover in our parser
- mark attributes with no value as parse errors and recover in our parser

I'm not sure what you mean. Before if our parser encountered syntax like this it blew up. It wouldn't return a result at all. So the fact that it returns a result and marks the syntax error shows that this both detects the problem and is capable of "moving on".
The scanner would be the place where you could get partial results, basically reading up to a point it'll still be OK until it finally hits the token it can't match.

Do you have an example input that you're thinking of?