Properly show locked file error message in web UI

[PVince81]

Whenever a file is locked, instead of hanging, the web UI needs to show a proper error message.

Currently the network console will return "500 Internal server error" and the log will show that there was a LockingException.

The LockingException needs to be caught properly in the ajax endpoints to be translated into a proper error message.

Needed to be able to show the locking exceptions:

• rename
• move
• new file
• new folder
• delete
• upload
• download
• getstoragestats

[PVince81]

This will require fixing all the ajax calls of the files web UI to properly catch exceptions and actually return them: (something which seems to have been missing for a long while)

[PVince81]

PR here #16718

[PVince81]

The PR above fixed many operations but not "Download".
Keeping open to address the "download" case which is more tricky.

[PVince81]

Download case here #17009

[DeepDiver1975]

getstoragestats

do we really need this? what's the impact? THX

[PVince81]

Probably not needed.
If that call fails because scanning failed, then the "max upload size" or "free space" will not be updated on the upload button, which isn't that bad.