Validate that the backend exists when setting the bucket type option 'backend'

[pma]

When creating a bucket type it's possible to set the backend option to one that does not exist.

Later any operation on that bucket type will just hang for a long time before failing, with no clear error. Perhaps its better to just fail, or at least warn, when the bucket type is being created. (tested with a put with the pb erlang client).

Noted: Detected this when I misstyped the backend name as set in the advanced.config file.

riak-admin bucket-type create reference '{"props":{"backend":"leveldb"}}'

advanced.config:

[
  {riak_kv, [
      {multi_backend_default, <<"bitcask">>},
      {multi_backend, [
          {<<"bitcask">>,  riak_kv_bitcask_backend, [
                           {data_root, "/var/lib/riak/bitcask/"},
                           {io_mode, erlang}
          ]},
          {<<"memory">>,  riak_kv_memory_backend, [
                           {max_memory, 100},
                           {ttl, 86400}
          ]},
          {<<"eleveldb">>, riak_kv_eleveldb_backend, [
                           {data_root, "/var/lib/riak/leveldb"},
                           {maximum_memory_percent, 30}
          ]}
      ]}
  ]}
].