Removed files stay in MRU lists even after failed opening them

[nielsm]

If you move/delete a file that's in one of the Recent lists and then try opening it from the Recent list, opening obviously fails. But the file stays on the Recent list even though it's no longer valid.

ADDITIONAL INFORMATION:
Most other applications remove dead files from MRU lists either when the list is about to be displayed, or when attempting to open it.
I would recommend the latter, because if any of the files in MRU are on removeable media (diskette, CD, network) stat()ing all of them might produce unreasonable delays.

[nielsm]

Some investigation and testing a (failed) fix shows that the MRU handling code is buried deep all over the place.
It might take some semi-serious reworking of the internal error/exception handling, to make it more structured, have less pure strings/chararrays and more typed exceptions. Only that way can really make it easy to catch file-unopenable errors etc and tell them apart from "user clicked cancel" etc.

[verm]

Again, very easily supported with the new MRU system, however I won't be making this change myself.

[plorkyeran]

(In [4717]) Make video and audio providers throw typed exceptions.

Don't display error messages and try other providers when the user
cancels loading a file.

Remove files from the MRU lists if they can't be found.

Closes #717.

[plorkyeran]

(In [4721]) Also remove timecodes and keyframes from the MRU lists when they cannot be found. Updates #717.

[verm]

Bump 2.2.0 tickets to milestone:3.0.0 (2.2.0 is becoming 3.0.0)