Support Returning Partial Results

[joe-elliott]

Is your feature request related to a problem? Please describe.
Occasionally we see intermittent failures to query traces due to a single block having issues. These issues could be data corruption or occassionally just GCS misbehaving. Currently this results in a 500 even though it's quite possible we have correctly retrieved the trace.

Describe the solution you'd like
In the above case let's return whatever we happen to have retrieved along with a simple warning. Something like:

Potential partial results. Failed to query block <block id>. 

Note: Will require some coordination with the Grafana team to add support for the warning.

[joe-elliott]

Thoughts:

1. Should be configurable to max number of blocks that can fail
2. Need to be able to alert/watch it