Pass through errors to client

[sushain97]

The format should be an errors key that contains a mapping from file name to error type and description.

For any type of query, store in memory a mapping from package to a mapping from filename to error. If an error occurs while handling a particular file, set the value. If it goes through cleanly, unset the value.

For an async query that triggers an execution of stats, return nothing. For an async query that has some results, return the current mapping of relevant ones minus the ones in progress (since those errors could be stale).

For a sync query, the errors can just be the ones from that execution of stats if one is required and otherwise the current mapping of relevant ones.