Cleaner output for bucket type creation errors

Could probably be further enhanced, but handles the problem reported by @jrwest in #893 much more gracefully.

````
$ ./riak-admin bucket-type create foo '{"props": {"r": "fred", "n_val": "abc", "consistent": "abc", "allow_mult": "x"}}'
Error creating bucket type foo:
r must be an integer or (one|quorum|all)
n_val must be an integer
allow_mult should be true or false
<<"abc">> is not a valid value for consistent. Use "true" or "false"
````