MB-100 Lower the severity of tap_estimate related error messages.
MB-100 Lower the severity of tap_estimate related error messages.

We send all the rebalance errors to the user visible log. Particularly
these error conditions don't really prevent rebalance from completing
successfully. Moreover, they are normal when mixed versions cluster is
being rebalanced. So to prevent spamming the user log we just lower
the severity of these messages.

Change-Id: Iaeb22470200b7a2cf6f96bba1428af30a43da8b4
Reviewed-on: http://review.couchbase.org/26437
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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