When slave executes a transaction bigger than slave's max_binlog_cache_size,
slave will crash. It is caused by the assert that server should only roll back
the statement but not the whole transaction if the error ER_TRANS_CACHE_FULL 
happens. But slave sql thread always rollbacks the whole transaction when
an error happens.
            
Ather this patch, we always clear any error set in sql thread(it is different
from the error in 'SHOW SLAVE STATUS') and it is cleared before rolling back
the transaction.