Multiple and vague error dialogues when opening non-locatable MusicXML via Open Recent

1. Open MusicXML.
2. Move its location.
3. Open the score via Open Recent.

Result: Dialogue appears with no explanation - just an exclamation mark in a yellow circle with the file location and two buttons: 'OK' and 'Quit'.

4. 'OK' or 'Quit'.

Result: Error dialogue appears saying it can't be opened, but without explanation.

Discussion: I propose to remove the first error dialogue that appears and in the remaining dialogue, state why it can't be opened. Also see this (particularly about the bullet points suggestion): #15989: Error message verbose and lacking capitalisation

Using MuseScore 2.0 Nightly Build (5531) - Mac 10.7.3.