[widgets][assetwidget] Add tooltip when asset is invalid

It follows the same idea as the experiment editor hinting.

Fixes #264
4 jobs for 264_add_tooltip_for_errors in 10 minutes and 25 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_36 #201055
docker

00:10:23

90.0%
passed build_linux_37 #201056
docker

00:10:13

90.0%
passed build_macosx_36 #201053
macosx

00:09:29

90.0%
passed build_macosx_37 #201054
macosx

00:09:05

90.0%