Give reason for failure to load instead of "please try again later"

Is your feature request related to a problem? Please describe.

Several bug reports such as this one #2778 are related to a failure to load Nextcloud office/collabora. The user is advised to "try again later", which makes little sense if there is a configuration error that won't go away by trying again later. In the case of #2278, Collabora refuses the connection because it does not know the WOPI host. In such a case , there should be a meaningful error message appearing in the browser, instead of the Microsoftish "try again later. This would, I think, prevent the users from just fumbling around in the dark.

Describe the solution you'd like

If the Collabora host refuses the connection, this should be said in the error message on screen.

Describe alternatives you've considered

Yes, sure, if you examine the nextcloud.log, you find a hint there, but the nextcloud log is rather overwhelming, you easily miss the forest for the trees. Therefore I'd prefer the simple and concise message I have suggested.

Additional context

Would also reduce the load on the bug tracking system, I guess.

Start by tracing the Collabora connection failure from the browser message and compare it with the related nextcloud.log entry. The work is done when a refused Collabora connection produces a concise, meaningful on-screen error instead of “try again later.”