Command "Beet: build" runs even though selected config file no longer exists

[OrangeUtan]

The build command uses the selected config file to execute beet.
However, it is still executed when that file doesn't exist, resulting in an error in the build task.

Instead of blindly executing the build command, checking if it exists would allow for more quality of live improvements:

• Notify the user that the selected config file doesn't exist anymore
• Open dialog to allow the user to choose a different config file

[vberlier]

Nice idea 👍