When attempting to export multiple photos (as local files, not to a service), if
there is some kind of IO error (e.g. disk full), you get a prompt which looks a
bit like this
Shotwell
Unable to export the following photo due to a file error.
<path>
Would you like to continue exporting?
[Continue] [Cancel]
If you press "Continue", (I believe) the behaviour is, it attempts to export the
next photo in the set you had selected.
However this is the exact behaviour that "Cancel" exhibits, too. So, to cancel the
whole export process, you must either mash Cancel N times, where N could be pretty
big, or get bored and run xkill(1) on Shotwell.