add ER return code

Author: rnro

Diff for: Sources/Memcache/MemcacheResponse.swift

@@ -22,13 +22,16 @@ struct MemcacheResponse {
         case NF
         case VA
         case EN
+        case ER
 
         init(_ bytes: UInt16) {
             switch bytes {
             case 0x4844:
                 self = .HD
             case 0x4E53:
                 self = .NS
+            case 0x4552:
+                self = .ER
             case 0x4558:
                 self = .EX
             case 0x4E46: