Add g_prefix_error_literal()

Because sometimes you don't want a lone "%s", and you don't
want the compiler yelling at you about format strings that
don't have any format in them.

Closes #663

docs/reference/glib/glib-sections.txt

@@ -786,6 +786,7 @@ g_set_error_literal
 g_propagate_error
 g_clear_error
 g_prefix_error
+g_prefix_error_literal
 g_propagate_prefixed_error
 <SUBSECTION>
 GErrorInitFunc

glib/gerror.c

@@ -1100,6 +1100,30 @@ g_prefix_error (GError      **err,
     }
 }
 
+/**
+ * g_prefix_error_literal:
+ * @err: (allow-none): a return location for a #GError, or %NULL
+ * @prefix: string to prefix @err with
+ *
+ * Prefixes @prefix to an existing error message. If @err or *@err is
+ * %NULL (i.e.: no error variable) then do nothing.
+ *
+ * Since: 2.70
+ */
+void
+g_prefix_error_literal (GError      **err,
+                        const gchar  *prefix)
+{
+  if (err && *err)
+    {
+      gchar *oldstring;
+
+      oldstring = (*err)->message;
+      (*err)->message = g_strconcat (prefix, oldstring, NULL);
+      g_free (oldstring);
+    }
+}
+
 /**
  * g_propagate_prefixed_error:
  * @dest: error return location

glib/gerror.h

@@ -244,6 +244,11 @@ void     g_prefix_error               (GError       **err,
                                        const gchar   *format,
                                        ...) G_GNUC_PRINTF (2, 3);
 
+/* if (err) prefix the string to the ->message */
+GLIB_AVAILABLE_IN_2_70
+void     g_prefix_error_literal       (GError       **err,
+                                       const gchar   *prefix);
+
 /* g_propagate_error then g_error_prefix on dest */
 GLIB_AVAILABLE_IN_ALL
 void     g_propagate_prefixed_error   (GError       **dest,