Fix uninitialized constant Redis::Error error

Author: junyuanz1

Diff for: lib/redis/pipeline.rb

@@ -58,7 +58,7 @@ def send_blocking_command(command, timeout, &block)
 
   class MultiConnection < PipelinedConnection
     def multi
-      raise Redis::Error, "Can't nest multi transaction"
+      raise Redis::BaseError, "Can't nest multi transaction"
     end
 
     private