Avoid an error if the file extension is not recognized.
Show an error panel when a render loop error occurs.

Also use the same error panel to indicate when an error occurred when dropping a file into CesiumViewer.
Use error panel for errors instead of alert.
Log errors to console by default when showRenderLoopErrors is true.
Adjust styling on error box.
Better error handling during widget startup.

Diff this change ignoring whitespace (on GitHub use ?w=1 on the URL).
Merge remote-tracking branch 'origin/master' into addErrorPanel
Merge remote-tracking branch 'origin/master' into addErrorPanel
Adjust CHANGES.md and allow errors to word-wrap.
Fix test which was failing do to CSS class name change.
Merge pull request #1063 from AnalyticalGraphicsInc/addErrorPanel

Show an error panel when a render loop error occurs.