Improve rebalance error reporting.

As far as I can tell, unknownNodes and {mismatch: 1} errors are there
for the same reason and depending on circumstances one or the other
can be returned. For example, if user passed an invalid node name that
happened to coincide with some existing atom, then "mismatch" error
would be returned. Otherwise, "unknownNodes" would be returned.

Our management UI handles only "mismatch" error and actually only this
error is documented in our API reference, so I removed the other
one. Even if any client was actually aware of this error, it must have
also been aware of the other one, so nothing should break.

Change-Id: I418d648806cb7f53732ec44f5384cf9a646e4fe2
Reviewed-on: http://review.couchbase.org/34271
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Give user at least some idea on why rebalance can't be started.

Change-Id: Id56e6e9f6d72f02e51904d60767b587ab8339eae
Reviewed-on: http://review.couchbase.org/34272
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Fix a typo.

Change-Id: Ia163b95ca73a79b6acfe8c9a362f15b1f32eba63
Reviewed-on: http://review.couchbase.org/34273
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Document /controller/rebalance as of pre delta recovery changes.

Change-Id: Iad68ce39e5951caba66b680b4945e75282dd3766
Reviewed-on: http://review.couchbase.org/34274
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Update api.txt according to delta recovery changes.

Change-Id: I3eb054535ef6b02b4056fd1b189c69c7b85fc40b
Reviewed-on: http://review.couchbase.org/34275
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>