Extend XMLStreamWriter validation test coverage

[ppalaga]

@cowtowncoder this is a preparation for the fix of #179:

• Assert specific validation error messages to get more certainty that the expected error occurred
• Extend XMLStreamWriter validation test coverage by checking the writer validation wherever we test the reader validation

[cowtowncoder]

Would be happy to merge this. Since there are non-test changes, one thing we need to have CLA (if not received yet, apologies if I missed it). One from Jackson repo:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

is one we use; it is usually easiest to print, fill & sign, scan/photo, email to cla at fasterxml dot com.

Once I have CLA it's good for all future contributions.

Looking forward to merging this, thank you!

[ppalaga]

we need to have CLA

I asked our legal team for an approval.

[cowtowncoder]

Thank you @ppalaga .

[cowtowncoder]

Since the only remaining changes here are test improvements, I can merge without CLA -- will do so.
(CLA needed for actual non-test code changes, just not test code)

[ppalaga]

@cowtowncoder our Legal team did not approve the CLA you pointed me to. The main concern lies in the broad scope of the copyright license, particularly with the request for joint ownership.
I wonder if I may let them review the corporate CLA?

[cowtowncoder]

Yes CCLA makes sense then. Have had a few company submit that, including some of FAANGs; no real difference for us.