Notify user of CSS syntax error

[ankit]

When editing CSS in the options page, if the entered CSS is syntactically wrong and user clicks save, the modal popup closes without saving the changes. Instead, the modal popup should notify the user that there was a syntax error in the entered CSS and should not close.

If at all possible, maybe also focus the line at which the error was encountered.

[rduenasf]

Are you checking this one out?

[ankit]

Yes, I'll do this one.

[rduenasf]

I'll take a look at #71 #65 and #20