patch 8.2.4010: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/if_python3.c b/src/if_python3.c
index 3bf387a..c8ca3c3 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -771,7 +771,7 @@
     if (python_loaded())
     {
 	if (verbose)
-	    emsg(_("E837: This Vim cannot execute :py3 after using :python"));
+	    emsg(_(e_this_vim_cannot_execute_py3_after_using_python));
 	return FAIL;
     }
 # endif