Improve error messages

[dantleech]

For example:

No explanation about what exception is about:

 Property dcms:foo in /dcms/sites/dantleech/bar

No parenthis around property name "format":

[PHPCR\RepositoryException] No default value for autocreated property format at /slinp/web/root/articles/Faster-than-light

Is a PathNotFoundException. Without looking at the code I don't really know what it is actually saying to me.

We should probably do an audit and clean up the error messages generally.

[dantleech]

Was just about to create this issue again, putting it on my todo :)

[dbu]

looks like some exceptions where expecting the reader to also see the exception class. definitely room for improvement.

[dbu]

just be careful about changing the exception type thrown - BC break risks, and PHPCR actually defines what exceptions to expect where (though we surely don't test that and there will be glitches currently)

[dantleech]

I was thinking just about changing the messagws to be "self sufficient" and being more helpful (like saying which choices are available in enum fields f e.)