Refine error handling mechanisms

[yaohaizh]

The test launcher/executor has mixed return error code and exception code. Unify the error handling mechanisms.

[bsaby]

Should handle errors instead of skipping tests.

Project: https://github.com/angelozerr/lsp4xml
Now: trigger tests from class/method level or codelens in following class file, tests are skipped.

Expect:
Show error to the user.

[jdneo]

Resolved in 0.15.0 and will release recently.