improve error reporting reducing the `try` extent

Hi, with this pull request i propose to reduce the extent of a `try` branch.

The current code was misleading for me. An error in CSS was reported as "module not installed". You may want to add another try catch, but it may mask errors coming from the optimizer. In my case i got a line number from the exception thrown inside `justDoIt`, and although it was false, i wouldn't like to mask it. I was able to reproduce the same error manually running `csso` on the concatenated file, and seeing the match helped me to know i was going in the right direction.