refs #6820 - catching apipie-bindings' MissingArgumentsError

depends on ~~Apipie/apipie-bindings#12~~
Sorry, I hadn't seen this.  I opened a PR against apipie-bindings earlier and put this in the exception message itself: https://github.com/Apipie/apipie-bindings/pull/16, otherwise all users have to catch the exception to get info out of it.

(Our installer was hitting a validation error and I couldn't see why.)