Improved diagnostics to report cases where FindBox is called without a literal
Converted these cases to errors

I added the filename to the error message to help users localize the issue.  In theory,
one could actually extract out token information and provide an even more detailed
diagnostic message to the user.
Tried to provide a good diagnostic error message
Merge pull request #30 from xogeny/master

Improved diagnostics for cases where FindBox is called without a literal