Fix the negative error number

[nogweii]

Description

This fixes the shellcheck error. Running on my laptop, exit -1 results in an actual return code of 255, so make that explicit.

Requirements / Checklist

• Read the Contributing Guidelines
• Read or at least glanced at the FAQ
• Read or at least glanced at the Wiki
• Scripts execute without error (if necessary):
  • If any of the scripts were modified they have been tested and execute without error, e.g.:
    • ./font-patcher Inconsolata.otf --fontawesome --octicons --pomicons
    • ./gotta-patch-em-all-font-patcher\!.sh Hermit
• Extended the README and documentation if necessary, e.g. You added a new font please update the table

What does this Pull Request (PR) do?

This should help with the Travis CI failures.

[ryanoasis]

Thanks good catch. Interesting, so it looks like it is actually 255 because it is 'out of range' of the error codes: http://tldp.org/LDP/abs/html/exitcodes.html

Perhaps we should just make it 1 ?

I can merge this tomorrow

[ryanoasis]

marking on 2.1.0 release, need to update changelog and all-contributors