filediff: Add a helper for showing user-criticial errors
The idea behind this decorator is that it should be used with functions where failure is important to the user (and may not be obvious otherwise). The only current use is on save, where it's possible to hit a traceback and have the callback handling crash out, but other than a log message or traceback on the command line... the user may not notice anything. This is made necessary by the GTK+/pygobject philosophy (which is generally extremely good) of treating errors of this kind as non-fatal. We also want it to not be fatal in this case, but... the user needs to know if they tried to save a file and it didn't work. On the other hand, if they tried to e.g., go to the next diff and we messed something up and didn't go to the next diff, (a) they'd notice and (b) showing them a modal dialog would be even more annoying than the actual error.
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