Handle invalid ext storage backend to keep mount point visible

[MorrisJobke]

Keep mount point visible and also ext storage config visible when
dealing with configs relating to storage backends or auth mechanisms
that were provided by an app that is currently disabled.

Downstream of owncloud/core#26539

Separate PR because @icewind1991 commented:

There is an OC\Files\Storage\FailedStorage that should probably bne reused instead

To test:

• checkout master at 6e3b8db or something similar
• setup external storage
• change to a master without a external storage or disable the app that provides the class (like the separate FTP external storage app)
• before: broken, because of unknown configuration
• after: working and showing that it is currently not connected - also the admin/user can delete the invalid config

[mention-bot]

@MorrisJobke, thanks for your PR! By analyzing the history of the files in this pull request, we identified @PVince81, @Xenopathic and @icewind1991 to be potential reviewers.

[codecov]

Codecov Report

Merging #6196 into master will increase coverage by 5.74%.
The diff coverage is 28.57%.

@@             Coverage Diff             @@
##             master   #6196      +/-   ##
===========================================
+ Coverage     31.45%   37.2%   +5.74%     
- Complexity    21210   21217       +7     
===========================================
  Files          1312    1312              
  Lines         79291   82432    +3141     
  Branches       1327    1329       +2     
===========================================
+ Hits          24938   30665    +5727     
+ Misses        54353   51767    -2586

Impacted Files	Coverage Δ	Complexity Δ
apps/files_external/js/settings.js	58.89% <28.57%> (-0.38%)	0 <0> (ø)
lib/private/Files/ObjectStore/Mapper.php	0% <0%> (-100%)	2% <0%> (ø)
lib/private/OCS/Provider.php	0% <0%> (-100%)	7% <0%> (ø)
...pFramework/Db/MultipleObjectsReturnedException.php	0% <0%> (-100%)	1% <0%> (ø)
core/Command/Integrity/SignApp.php	0% <0%> (-100%)	9% <0%> (ø)
...b/private/App/AppStore/Bundles/GroupwareBundle.php	0% <0%> (-100%)	2% <0%> (ø)
apps/files_trashbin/appinfo/app.php	0% <0%> (-100%)	0% <0%> (ø)
lib/private/App/CodeChecker/EmptyCheck.php	0% <0%> (-100%)	6% <0%> (ø)
apps/dav/lib/Capabilities.php	0% <0%> (-100%)	1% <0%> (ø)
.../private/App/AppStore/Bundles/EnterpriseBundle.php	0% <0%> (-100%)	2% <0%> (ø)
... and 804 more

[MorrisJobke]

@icewind1991 Updated as discussed.

[icewind1991]

enhanced it to show the failed item in red in the filelist like other invalid external storages

[icewind1991]

👍 for morris' part

[MorrisJobke]

👍 for @icewind1991's part