[Ada] Change message format on missing return

[Pierre-Marie de Rodat via Gcc-patches]

To make it compatible with -gnatwE so that this warning is easier to
spot when needed.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * sem_ch6.adb (Check_Returns): Change message on missing return.
diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -7760,7 +7760,7 @@ package body Sem_Ch6 is
                  ("RETURN statement missing following this statement<<!",
                   Last_Stm);
                Error_Msg_N
-                 ("\Program_Error ]<<!", Last_Stm);
+                 ("\Program_Error [<<!", Last_Stm);
             end if;
 
             --  Note: we set Err even though we have not issued a warning