[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 Job ID Name Coverage
  Build
passed #201055
docker
build_linux_36

00:10:23

90.0%
passed #201056
docker
build_linux_37

00:10:13

90.0%
passed #201053
macosx
build_macosx_36

00:09:29

90.0%
passed #201054
macosx
build_macosx_37

00:09:05

90.0%