Unhelpful error on Arg parse

[mbStavola]

For context, this is what was being read in from_yaml():

- es:
  short: e
  help: Sets the ES tag on the output files.
  default_value: 001
  takes_value: true

When running my program I got the very confusing:
thread 'main' panicked at 'called Option::unwrap()on aNone value', ../src/libcore/option.rs:325

Looking at the backtrace, I was able to narrow it down to a Arg::from_yaml() not parsing correctly. But no idea where or why!

The fix was easy: simply wrap 001 in quotes to make it a string. However, it was a bit annoying to figure it out.

It would be nice to see the context of the panic!() in these cases. For example, what arg setting failed.

[kbknapp]

Thanks for filing this! I agree, I think adding an expect instead of unwrap will fix this. Once I get through the existing PRs I'll fix this one as well.