qga: Improve error for guest-set-user-password parameter @crypted

The Windows version of guest-set-user-password rejects argument
"crypted": true with the rather useless "this feature or command is
not currently supported".  Improve to "'crypted' must be off on this
host".

QERR_UNSUPPORTED is now unused.  Drop.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240911131206.2503035-3-armbru@redhat.com>
diff --git a/include/qapi/qmp/qerror.h b/include/qapi/qmp/qerror.h
index 38e8976..101c114 100644
--- a/include/qapi/qmp/qerror.h
+++ b/include/qapi/qmp/qerror.h
@@ -26,7 +26,4 @@
 #define QERR_PROPERTY_VALUE_OUT_OF_RANGE \
     "Property %s.%s doesn't take value %" PRId64 " (minimum: %" PRId64 ", maximum: %" PRId64 ")"
 
-#define QERR_UNSUPPORTED \
-    "this feature or command is not currently supported"
-
 #endif /* QERROR_H */
diff --git a/qga/commands-win32.c b/qga/commands-win32.c
index 61b36da..038beb8 100644
--- a/qga/commands-win32.c
+++ b/qga/commands-win32.c
@@ -1914,7 +1914,7 @@
     GError *gerr = NULL;
 
     if (crypted) {
-        error_setg(errp, QERR_UNSUPPORTED);
+        error_setg(errp, "'crypted' must be off on this host");
         return;
     }