improve UX when encountering missing/invalid library folder

[ronso0]

Amongst others, my library contains a folder on an external drive.
That drive was not mounted when I wanted to add a subfolder of my home directory. Adding was refused, message says it's already in the library or the library is being scanned. Neither was true.
That was driving me crazy so I removed all watched folders to finally figur it's failing due to the unmounted drive :\

the message
https://github.com/mixxxdj/mixxx/blob/3546054e299956a952c1fb1b7855e95e5f2be200/src/library/library.cpp#L504

caused by
https://github.com/mixxxdj/mixxx/blob/3546054e299956a952c1fb1b7855e95e5f2be200/src/library/dao/directorydao.cpp#L68

The logged line is clear about the cause:
Aborting to add QFileInfo(/home/abc/Music/mmumu) : Loaded directory QFileInfo(/mnt/yxo/audio) does not exist or is inaccessible

After closing the dialog and hitting Apply in the preferences Mixxx says "one or more directories were added... Rescan / Cancel"
Also wrong.

____________________________________________________

1) can DirectoryDAO::AddResult be passed through to the requester
   so that we can have explicit error messages?

2) Why is a missing directory hindering the addition of others in the
   first place? I'm aware the dir is missing, that was the reason to add
   another track source ;)

[ronso0 (ronso0)]

only affects main/2.4, 2.3 does handle this case satisfactory.

[ronso0 (ronso0)]

Re 1) pass DirectoryDAO::AddResult through to the requester
Is there a smarter way than adding identical enums to trackcollection and trackcollectionmanager?

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/10481