Fixes IE8 not letting you get the imports on some style sheets

For some reason, IE8 seems to throw a 'Permission denied' exception when trying to access the imports of some style sheets. For me this only happens on style sheets which are included into document header from add-ons by gwt.

I could not understand why you aren't permitted to access the imports of these style sheets while all the others work - they're loaded from the same server anyway.

The fix adds exception handling and logging for the IE8-specific import handling, allowing the process to continue even if such a style sheet is encountered.