medialibrary.ui: Use a smaller, symbolic label for warning_infobar close button

The use of the "Close" label made the button a bit too prominent, especially
in languages such as French. Besides, one can argue that the verb "close"
may be weird in the context of an infobar inside another widget.
Using the "✖" unicode symbol and a tooltip instead.
medialibrary: Show the amount of errors with ngettext in warning_infobar

Until now we just said "Errors occurred while importing", so we might as well
say the exact number to let the user know how many clips failed to import.

This is also demonstrates Python's new standard way of formatting strings.
Update French translation and improve the French AppData.xml description