Natanael Arndt authored Jul 25, 2018 and Thibault Saunier committed Jul 25, 2018

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