Add 0x to indicate a hex number

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/network/protocol/tim/packet/login/PasswordSubmission.kt

@@ -168,7 +168,7 @@ object SubmitPasswordPacket : PacketFactory<SubmitPasswordPacket.LoginResponse,
                         }
                         else -> null
                     }
-                        ?: error("Unknown length flag: " + flagFront.toUByte().toUHexString() + flagBack.toUByte().toUHexString() + ", remaining packet = ${readBytes().toUHexString()}")
+                        ?: error("Unknown length flag: 0x" + flagFront.toUByte().toUHexString() + flagBack.toUByte().toUHexString() + ", remaining packet = ${readBytes().toUHexString()}")
                 )
 
                 discardExact(23 + 3)//01 D3 00 01 00 16 00 00 00 01 00 00 00 64 00 00 0D DE 00 09 3A 80 00