Blender Git Loki

Asset Bundle Copy button: only report each external dependency once

The `ASSET_OT_bundle_install` operator only works when the blend file is
self-contained. It reports any external dependencies. Before this patch:

- every dependency was mentioned, even when it repeated the same
filename over and over again, and
- multiple dependencies were all mentioned in the error popup,
potentially filling the screen.

This is now resolved by:

- only reporting each external file once, and
- referring to the console when there are multiple external dependencies.

Reviewed by: severin, dfelinto

Differential Revision: https://developer.blender.org/D13413