[util] Return diff values for diff failures

Need documentation in the help output...

Fixes https://github.com/harfbuzz/harfbuzz/issues/5164

util/face-options.hh

@@ -110,6 +110,7 @@ face_options_t::post_parse (GError **error)
     {
       g_set_error (error, G_OPTION_ERROR, G_OPTION_ERROR_FAILED,
 		   "%s: Failed loading font face", font_path);
+      return_value = RETURN_VALUE_FACE_LOAD_FAILED;
       return;
     }
   }

util/font-options.hh

@@ -110,6 +110,7 @@ font_options_t::post_parse (GError **error)
   {
     if (!hb_font_set_funcs_using (font, font_funcs))
     {
+      return_value = RETURN_VALUE_FONT_FUNCS_FAILED;
       const char **supported_font_funcs = hb_font_list_funcs ();
       if (unlikely (!supported_font_funcs[0]))
       {

util/main-font-text.hh

@@ -43,6 +43,7 @@ struct main_font_text_t :
   int operator () (int argc, char **argv)
   {
     add_options ();
+
     parse (&argc, &argv);
 
     this->init (this);
@@ -52,7 +53,10 @@ struct main_font_text_t :
 
     this->finish (this);
 
-    return this->failed ? 1 : 0;
+    if (this->failed && return_value == RETURN_VALUE_SUCCESS)
+      return_value = RETURN_VALUE_OPERATION_FAILED;
+
+    return return_value;
   }
 
   protected:

util/options.hh

@@ -54,6 +54,14 @@
 #include <glib/gprintf.h>
 
 
+enum return_value_t
+{
+  RETURN_VALUE_SUCCESS = 0,
+  RETURN_VALUE_OPTION_PARSING_FAILED = 1,
+  RETURN_VALUE_FACE_LOAD_FAILED = 2,
+  RETURN_VALUE_OPERATION_FAILED = 3,
+  RETURN_VALUE_FONT_FUNCS_FAILED = 4,
+} return_value = RETURN_VALUE_SUCCESS;
 
 static inline void fail (hb_bool_t suggest_help, const char *format, ...) G_GNUC_NORETURN G_GNUC_PRINTF (2, 3);
 
@@ -71,7 +79,10 @@ fail (hb_bool_t suggest_help, const char *format, ...)
   if (suggest_help)
     g_printerr ("Try `%s --help' for more information.\n", prgname);
 
-  exit (1);
+  if (return_value == RETURN_VALUE_SUCCESS)
+    return_value = RETURN_VALUE_OPTION_PARSING_FAILED;
+
+  exit (return_value);
 }
 
 struct option_parser_t