Relayer stats visualizations and more granular error classification

[polymorpher]

See #290 and follow up comments. Currently, no information is given for cause of failures in the metric APIs. We need more visibility into whether the errors are due to incorrect request parameters (user or client error), authentication failure (user error), network error, RPC error, or relayer internal error. The information is already stored in ElasticSearch and should be retrievable using some carefully designed queries.