Cluster: don't allow GET option in cluster mode.
Cluster: don't allow GET option in cluster mode.

The commit also refactors a bit the error handling during SORT option
parsing.

Hailei/redis,Hailei/redis,slfs007/mk-redis,jxwr/redis,xuzhezhaozhao/redis_reading,ksarch-saas/redis,ttuna/msot-redis,xuzhezhaozhao/redis_reading,spearhead-ea/redis,HunanTV/redis,splitice/redis,maodeyi/redis_lua,HunanTV/redis,MSOpenTech/redis,louisliangjun/redis,splitice/redis,himoca/redis,MSOpenTech/redis,ttuna/msot-redis,HunanTV/redis,louisliangjun/redis,darksideofthemoo/redis-histogram,himoca/redis,xuzhezhaozhao/redis_reading,louisliangjun/redis,himoca/redis,maodeyi/redis_lua,xuzhezhaozhao/redis_reading,ksarch-saas/redis,ttuna/msot-redis,splitice/redis,darksideofthemoo/redis-histogram,louisliangjun/redis,spearhead-ea/redis,darksideofthemoo/redis-histogram,ttuna/msot-redis,ksarch-saas/redis,darksideofthemoo/redis-histogram,MSOpenTech/redis,splitice/redis,jxwr/redis,Hailei/redis,ttuna/msot-redis,jxwr/redis,Hailei/redis,ramonsnir/redis,MSOpenTech/redis,ramonsnir/redis,HunanTV/redis,maodeyi/redis_lua,slfs007/mk-redis,spearhead-ea/redis,ksarch-saas/redis,slfs007/mk-redis,ramonsnir/redis,MSOpenTech/redis