Improve logging and error messages

[brizental]

This needs a little bit more digging, but there are some log/error messages that could be improved.

Like, if we try and send in a request with missing params, it will return the following log:

{"Logger":"fxa_email_service-1.115.0","Type":"fxa_email_service:log","Pid":91132,"Severity":3,"Timestamp":1530658150217212000,"Fields":{"msg":"{\"code\":\"400\",\"errno\":\"101\",\"error\":\"Bad Request\",\"message\":\"Missing email params.\"}","agent":"curl/7.54.0","remoteAddressChain":"127.0.0.1","path":"/send","method":"POST"}}

Here, it would be nicer to know, for example, which are the missing params. In auth server we just add more properties to some of our error messages when they need more info (https://github.com/mozilla/fxa-auth-server/blob/master/docs/api.md#response-format), that could be a solution here.

Anyways, probably not something urgent, but I just noticed this and thought would be best to document it.

[philbooth]

Ah, yeah, there used to be a TODO comment about this but I can't find it now, so thanks for raising this!

Anyway, for the validation errors, I think it depends on a resolution to rwf2/Rocket#596 happening first.