feat: query result messages

[alanshaw]

From convo in discord:

The indexing service should always return a query result, even if an error is encountered.

An error should be a field in the returned query result.

It would allow the service to return what it found, even if it encountered errors in follow on jobs.

It also prevents a bad storage node from eclipsing results for data stored on a good node.

Contrived example: a node stores some data but then goes offline, but it was replicated to another node - the error for not being able to fetch a shard from the offline node does not prevent the query from completing...

It aids debugging - you can get a partial result and an error that would allow you to determine how far the indexer got with the query.

This PR adds a "messages" property to the query result that allows warnings and other information to be returned in a query result. It also alters the service to return non-fatal errors in this field.