[registry] malformed tag parameters silently fail

[jamiealquiza]

When applying a tag filter via curl (behavior with grpc hasn't been reviewed), incorrectly formed tag parameters will not emit an error. This may result in a user unexpected receiving a full object list that includes resources that do not match the intended tags.

Example, incorrectly formed param ("tags" vs "tag") and response, no errors:

$ curl -s "registry/v1/brokers/list?tags:topicmappr_map=pool1" | jq .
{
  "ids": [
    10000,
    10001,
    10002,
    10003,
    10004,
    10005,
    10006,
    10007,
    10008,
    10009,
    10010,
    10011
  ],
  "brokers": {}
}

Correctly formed, different results:

$ curl -s "registry/v1/brokers/list?tag=topicmappr_map:pool1" | jq .
{
  "ids": [
    10006,
    10007,
    10008,
    10009,
    10010,
    10011
  ],
  "brokers": {}
}