Empty error messages are sometimes returned

[kierdavis]

Some work, but others don't. I haven't tested this thoroughly.

# led foo
- Error: unknown argument
# gpio-read foo
- 

[PeterJCLaw]

This sounds similar to an issue I hit playing with my arduino the other day.

In that case, I had found that removing either of the following resolved the issue:

• my added a ninth command (I was testing the echo command I'd documented in Add documentation for custom arduino commands docs#87)
• call gpio-write (which make_safe does)