web: Add image and css for play and stop buttons

These are based of the patternfly btn-default class, but changed
so that they look different and have an image. They're an appropriate
size.

Added a table cell class for containing these buttons, since we want
less padding around them.

Part of fix for Issue #301
Closes #307

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
web: Don't display Unhandled exceptions in a dialog

These go to the console, and are not useful for users to see
in a dialog. Show an 'Ooops!' in the status bar instead, and
suggest that the user restarts Cockpit, when they click on the
oops.

This method catches all of the unhandled exceptions, not just
those we were wrapping in a besafe() function.