improve readability of error "Unable to generate a signwriter image with font at size Array"

Hi,

my site is generating the error "Unable to generate a signwriter image with font at size Array" The error is rather cryptic -- it's hard to figure out what it means and what can be done to fix the problem. Perhaps it might be a good idea to make it easier to understand what that error means.

Everything works otherwise, and the images are properly generated. Thanks for your module!

Albert.