Default icon when invalid ones are used

[Andre601]

There is currently a nasty issue where the build will fail when an invalid icon has been defined.

As an example: FontAwesome did - as they usually do - change some icon names. Namely github-square has been renamed.
This would now cause builds to fail due to some template issue or similar.

If possible, should Material for MkDocs default to a fallback icon when an invalid one is found, so that it doesn't fail the build.