REST POST /pools/default/buckets error message improvements

Summary: A couple of the error messages when using the REST API endpoint for creating buckets are a little confusing/misleading.

Scenario 1:

When I POST a completely empty body to /pools/default/buckets, the error message returned is "errors": { "authType": "invalid authType" }. I didn't specify an authType so this could be confusing (especially if I put authType in the wrong place, like in the URL).

Suggestion: Change the message to say "you must specify an authType and it must be a valid authType" or alternatively, if authType is not POSTed correctly, return error "you must specify an authType" and only return "invalid authType" if an authtype was specified and not valid.

Scenario 2:

When I POST only authType = "none", one of the error messages is "proxyPort": "Port must be a number.". This could be similarly confusing as in scenario 1. I would recommend "you must specify a proxyPort and it must be greater than 1023 and less than 65536" or "you must specify a proxyPort".