CCBC-536: lcb_durability_poll should return error on schedule failure

Previously this was ignoring the return value of `mctx->done()`, which
could return with an error code.

Change-Id: I2d133a0e1cb29f271f7397e1e4cf463a63b0d010
Reviewed-on: http://review.couchbase.org/42643
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>