[BUGFIX] RTE: Insert Tag dialogue may raise security error in Firefox

Problem: If a style sheet from another domain is imported by the style
sheet configured for the RTE, a security error is raised by Firefox
when opening the Insert tag dialogue window.
Solution: Catch the error and report it on the console.

Releases: master, 6.2
Resolves: #62316
Change-Id: I093416abbc784f7548cda3b6c9be1d4b7d484040
Reviewed-on: http://review.typo3.org/33375
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>