Improve error message when collection name omitted

[Yuqi-Du]

What this PR does:
JSON API should detect the mismatch of provided command with url path.

e.g. 1
{{base_json_url}}{{json_api}}/namespace

{
    "find":{}
}

should prompt No 'find' command found as NamespaceCommand

e.g. 2
{{base_json_url}}{{json_api}}/namespace/collection

{
    "randomGuy":{}
}

should prompt No 'randomGuy' command found as CollectionCommand

e.g. 3
{{base_json_url}}{{json_api}}/namespace/collection

{
    "createNamespace":{}
}

should prompt No 'createNamespace' command found as CollectionCommand

Which issue(s) this PR fixes:
Fixes #800

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CLA Signed: DataStax CLA

[tatu-at-datastax]

Looks good! The only thing is that I think (as per note I added) it'd make sense to extract DeserializationProblemHandler to reuse in unit test.

[vkarpov15]

LGTM 👍