vbucketmigrator needs to handle TAP_MUTATION error responses

Bugging this up so we don't forget. In this bug, vbucketmigrator might need improvements to handle error responses from TAP_MUTATION. Code inspection shows it doesn't and seems to drop them on the floor. email trail...

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> If a replica receives a TAP_MUTATION but has hit the same kind of mem-used > mem-quota error (in MB-1820), what happens? As a thought experiment, if the codepaths are the same as with MB-1820, perhaps errors are returned to the tap stream sender/source? Does something reasonable happen in the tap stream sender/source (I suppose that's the vbucketmigrator) to retry?

That is correct – it will error and vbucketmigrator will receive that error. Edge cases are a pain, but it does seem safer to overshoot the memory than to just stop being able to replicate.