Improve Result's Display impl for extension values

Previously, the `Display` impl for `vk::Result` did not include handling
for extension values. For example, VK_ERROR_OUT_OF_DATE_KHR would
display simply as "-1000001004". Now, we fall back to the `Debug` impl
of the `Result` if the value is unknown (i.e. from an extension). This
preserves the current nice messages for non-extension values, and the
numeric output for truly unknown values, but displays the enum variant
name (e.g. "ERROR_OUT_OF_DATE")  for extension values.

ash/src/vk/enums.rs

@@ -997,7 +997,7 @@ impl fmt::Display for Result {
         if let Some(x) = name {
             fmt.write_str(x)
         } else {
-            self.0.fmt(fmt)
+            write!(fmt, "{:?}", &self)
         }
     }
 }

generator/src/lib.rs

@@ -1535,7 +1535,7 @@ pub fn generate_result(ident: Ident, enum_: &vk_parse::Enums) -> TokenStream {
                 if let Some(x) = name {
                     fmt.write_str(x)
                 } else {
-                    self.0.fmt(fmt)
+                    write!(fmt, "{:?}", &self)
                 }
             }
         }