[Issue #10] Repair setting a sticker for the first time.

In MPD commit 432675d4c23450c527333ed4a29d13f843e001da the error
message for "sticker get" when the sticker is not found was changed.
mpdpopm (foolishly) relied on the error message. This patch changes
the salient logic to examine only the error code, instead.

mpdpopm/src/clients.rs

@@ -676,8 +676,8 @@ impl Client {
                 }
             })?))
         } else {
-            (text.starts_with("ACK ") && text.ends_with("no such sticker\n"))
-                .context(Operation::GetSticker, &msg)?;
+            // ACK_ERROR_NO_EXIST = 50 (Ack.hxx:17)
+            (text.starts_with("ACK [50@0]")).context(Operation::GetSticker, &msg)?;
             Ok(None)
         }
     }