Add deeper XML checking to update-translation script

- Catch problems such as mismatched formatting characters. Remove
  messages that can give problems at runtime.

- Also remove unfinished/untranslated messages, they just take up space
  in the ts and waste parsing time.

Fixes #4774.
Update translations after update script improvements
Merge pull request #4776

6f5d33b Update translations after update script improvements (Wladimir J. van der Laan)
da59f28 Add deeper XML checking to update-translation script (Wladimir J. van der Laan)