The "Unknown error" message is totally helpless

[kirisakow]

Dear maintainers (and everyone who wishes all best to this fantastic project),

The "Unknown error" message in Web UI and Seafile-Drive is totally helpless: one nasty issue #2632 took me more than a month to fix.

This is sad and waste of time. As such,

1. Either the message should be appended with a clue, even a hypothetical one. For example: Unknown error. If you use a Nginx reverse proxy, make sure you have properly set the "client_max_body_size" option. See https://github.com/haiwen/seafile/issues/2632 for details.
2. Or it should have a unique error number, so that users can Google it and quickly find a fix in the documentation. For example: Error 20230114. Please refer to the documentation.

Thank you