better error message on too-large chunked response [risk: low]

[davidangb]

DataBiosphere/firecloud-app#83

if orch receives a chunked response from a subsystem, and that response is over the response-chunk-aggregation-limit size limit, we previously returned an error message that was confusing to end users: "sendReceive doesn't support chunked responses, try sendTo instead"

This PR attempts to make that error message more understandable.

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[coveralls]

Coverage decreased (-0.05%) to 67.045% when pulling 3119d7c on da_83_chunkError into 00143d1 on develop.

[rtitle]

Not a big fan of parsing error message strings as it's unreliable (and will break if/when we upgrade to akka-http). But I don't really have a better alternative, and I agree the new error message is much better. 👍

[davidangb]

Thanks - I agree it's unreliable in the face of upgrades, but the "failure" case in this code is to fall back to the current behavior of case Failure(error) => ... we'd simply stop rewording this particular error.