Give reasons in error message when importing a playlist fails

[edwinst]

Description

The app current gives a very general message like "playlist could not be imported" if there are any problems when importing a playlist. It would help a lot with troubleshooting if the app gave the reason why the playlist could not be imported. By trial and error I found the following problems in my playlist files:

• playlist uses absolute paths that do not exist on the phone
• playlist uses correct relative paths but at least one of the audio files does not exist on the phone
• playlist is missing some or all "#EXTINF:" lines (I thought they were optional, but if they aren't, a specific error message would be great)
• playlist has Mac-style line-ends (only \r from iTunes export instead of \n or \r\n) (I don't know if this alone caused the import to fail or if it was a combination with the above)

Problem solved

The problem is that currently the user has to guess what is wrong with a playlist file if the import fails.

Other implementations

I don't think other implementations are relevant here, as the details of the error messages really depend on the Auxio import code.

Benefit

Giving reasons in the error message speeds up troubleshooting, improves user experience, and reduces need for user support.

Duplicates

• [X] I have checked this feature request for duplicates.
• [X] I have checked that this feature is not implemented in the lastest version.
• [X] I have checked the Why Are These Features Missing? page.
• [X] I agree to the Contribution Guidelines.

[OxygenCobalt]

This is kind of a duplicate of #739. The issue is that this requires a more complicated indicator (i.e a Snackbar), which means another view component I have to cram into the main UI. This is frustrating since I have no clue what will actually happen. Merging and adding this to the issue.