Fix errors handle on plan create

[franjid]

Summary

Fix for https://github.com/braintree/braintree_ruby/issues/223

Checklist

• [x] Added changelog entry (If there isn't an #unreleased section, add that and your changelog entry to the top of the changelog)
• [x] Ran unit tests (rake test:unit)

[crookedneighbor]

Thanks for catching this. I want to review with the API team who added this to get some more context.

[hollabaq86]

for internal tracking, ticket 1905