Fix markup of error message in german translation

The bould-tag (<b>) was was not closed (</b>) in the German translation of the message string with the msgid starting with: "Pitivi detected a serious backend problem ..." And thus the text could not be set for the error-dialog.

This commit adds the closing-bould-tag and fixes #2224 (closed)

Edited by Natanael Arndt

Merge request reports