client/Response: reimplement Error() without FmtError()

With libfmt versions older than 7, this leads to an endless recursion
between Error() and FmtError(), resulting in a crash due to stack
overflow.  D'oh!

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1295

NEWS

@@ -1,6 +1,7 @@
 ver 0.23.2 (not yet released)
 * neighbor
   - mention failed plugin name in error message
+* fix crash with libfmt versions older than 7
 
 ver 0.23.1 (2021/10/19)
 * protocol

src/client/Response.cxx

@@ -66,7 +66,11 @@ Response::WriteBinary(ConstBuffer<void> payload) noexcept
 void
 Response::Error(enum ack code, const char *msg) noexcept
 {
-	FmtError(code, FMT_STRING("{}"), msg);
+	Fmt(FMT_STRING("ACK [{}@{}] {{{}}} "),
+	    (int)code, list_index, command);
+
+	Write(msg);
+	Write("\n");
 }
 
 void