bladeRF-cli: Handle unterminated quote as a non-fatal error condition

And additional return value has been added to str2args in order to
differentiate between an unexpected fatal (e.g, memory) error and
unterminated quotes.

This was previously causing cmd_handle() to fall through to an
"Unknown error" case, which is treated as a fatal error, resulting in
the CLI being terminated.

Thank you to Jens Elofsson (jenselo) for tracking down and reporting the
underlying defect! (Issue #259)