Add a hint to error responses on fetch API

[skandragon]

Currently, a response like

data: {"type":"error","message":"IllegalStateException: unknown word ':eqx'"}

is generated for many reasons, some of which retries would work, some they would not.

It would be useful to add an indicator showing if a retry could be successful, so we can easily determine if we need to bounce failure to the user of Kayenta, or if we should internally retry. We could default to retrying, so only hard errors on the query syntax itself need to be decorated initially.

In all forms of error cases, a 200 response is given, so we cannot use that today. That would be another option, but the JSON format is easier to handle in our current code.

[pparth]

+1.
This is very important to us too. We use a custom Atlas Query Editor and it would be great to have this feature.