[3.x] Add readable export errors.

[bruvzg]

Adds readable errors to the export dialog and log, instead of the misleading missing/corrupted templates or configuration error error that's currently displayed.

• Add custom error popup, with scrollable RTL and color coded warnings and errors, to improve readability.
• Make sure all export plugins use the same error reporting interface (currently there's a mess of random prints, and multiple "warning" and "I/O error" dialogs).
• Add missing error messages.

Before:

After ("Export All", for single export it's displayed only if there's something to show):

Fixes #61392

[Zireael07]

Can we expect something similar for master/4.x?

[bruvzg]

Can we expect something similar for master/4.x?

Definitely, I'll port it to master as soon as 3.x version is done.

[akien-mga]

Thanks!