Checking if files were writeable created files in the process. In the
case where someone adds a list of N files to dangerzone but exits before
converting, they would be left with N 0-byte files for the -safe
version. Now they don't.
Fixes#214
All filename-related exceptions were of class DocumentFilenameException.
This made it difficult to disambiguate them. Specializing them makes it
it easier for tests to detect which exception in particular we want to
verify.
The document's state update is better update in the convert() function.
This is because this function is always called for the conversion
progress regardless of the frontend.
