Add message to fonts.len() == 0 assert

If msttcorefonts isn't installed, output a more informative error message.

./servo ../src/test/html/about-mozilla.html prints
           "task 'LayoutWorker' failed at 'No matching font(s), are the appropriate fonts installed?',
            ../servo/src/components/gfx/font_context.rs:195"
vs. 
            "task 'LayoutWorker' failed at 'assertion failed: fonts.len() > 0',
            ../servo/src/components/gfx/font_context.rs:195"