redis.tcl: return I/O error message when peer closes connection.

tests/support/redis.tcl

@@ -170,7 +170,10 @@ proc ::redis::redis_read_reply fd {
         - {return -code error [redis_read_line $fd]}
         $ {redis_bulk_read $fd}
         * {redis_multi_bulk_read $fd}
-        default {return -code error "Bad protocol, '$type' as reply type byte"}
+        default {
+            if {$type eq {}} {return -code error "I/O error reading reply"}
+            return -code error "Bad protocol, '$type' as reply type byte"
+        }
     }
 }