bad error message when command line option mistyped

[diekhans]

A collaborator accidentally left one of the `-' off of an option like this:

gffread --ids H0C.ids -gtf isoforms.gtf > H0C.gtf

and got the deceptive error message:

Error: file/directory tf does not exist!

As they are a lab biologist who is a beginner at bioinformatics, this wasted a lot of time.

Please always include what file an error occurred on.