Add check of xml file to plain text export.

I just git bitten by issue #342.
Fortunately I was able to repair the corrupted backup by hand and did not loose any data.
But this could cause a normal user quite a shock, so I think it is better to check the backup automatically right after the export and tell the user if it failed.
Unfortunately I don't have the original data anymore that produced the corrupted file on export.
But I'll have a look if I can reproduce it and maybe also fix the actual cause of the bug...