Show server error messages in modal dialog to bypass autoclose of main window

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## Feature request

**Which Nextcloud Version are you currently using:**
3.15.3

**Is your feature request related to a problem? Please describe.**
When there is an error, the main dialog allows me to see additional details, such as the error message sent by the server.
However, because the main dialog is built to always close when it loses focus, it's practically impossible to keep that error message visible for context while doing anything else on the machine.

**Describe the solution you'd like**
Disabling autoclose would be bound to confuse users in one way or another. So I guess the best option would be to make it so that error details are opened as a separate model dialog with the usual window decorations. This one can stay open and be closed manually by the user.

**Describe alternatives you've considered**
- Copy-and-paste the error message: Cannot select text to copy the message. Instead a weird touch-like gesture to switch tabs happens.

No files, tests, or entry points are named. Start by tracing the desktop client's error-details dialog and the path that displays server errors; done means the details open in a separately closable decorated dialog and the message can be selected and copied without the main window autoclosing.

Written by the indexing model from the issue text.