Display better warnings for non-native formats unable to be found when opening via Open Recent

1. Open supported by MuseScore that isn't mscz or mscx (e.g. MusicXML).
2. 'Close'.
3. Relocate the file.
4. Open the file via 'Open Recent'.

Desired result: A dialogue appears: 'The following file(s) cannot be located:' with a list of files.
Actual result: A dialogue appears: 'Cannot read file ...: could not open MusicXML file'.

Note: This also applies to MIDI (with the same warning, if opened after a MusicXML) - maybe other formats too?

Using MuseScore 2.0 Nightly Build (513d2af) - Mac 10.7.5