Fix IPTC Keyword reading for real

Back in f855b0999f1101ea112f3e1ad42d4db5e1fc3714 I removed too much.

(cherry picked from commit 91c6f915b3af007989b83f148817d7bf3ad51dca)
Look for invalid UTF8 in tags on startup

I didn't put this into a db update step since it could be introduced via
XMPs at any time.

(cherry picked from commit 7982116587ddc4809dd7615396f04195df9c9b6e)