fast-export/import: cleanups and translation

In a previous v2 patch series[1] that I sent last May, there were two preparatory cleanup patches[2][3] that have been dropped in the v3 and next versions. I think these two cleanup patches are worth resending in their own series though.

While at cleaning things up, I realized that, when working in this area of the code, I have often been annoyed by the fact that few error and warning messages were marked for translation. So I decided to also address this here.

So patches 1/5 and 2/5 are small code cleanups that are resent, while patches 3/5, 4/5 and 5/5 are about marking strings for translation.

This is related to #519.