Rename somewhat confusing variable names in ApiErrorReporter

This is a direct follow-up to the comments in patch
Ibb169101bd75e4244ba7cae404ccaf49b9b3171d.

I think the message "wikibase-validator-invalid": "Invalid value"
is supposed to be part of an other message and therefor should not
have punctuation.

Change-Id: Iaa22c350b1e563c65d93dee8f1d18915e514574e
Merge "Rename somewhat confusing variable names in ApiErrorReporter"