Check for a valid file created from tempnam()

I was getting a weird "file not found" error from the uglifycss filter. After searching through the code, I found that my tempdir was not writeable by PHP due to a permissions problem. tempnam() returns false when a file could not be created for writing as in this scenario. If the error is not caught, the uglifycss command runs with a missing argument (the filename). Suggesting a simple exception to help troubleshooting.