Obtain DispatchError::Module info dynamically (#453)

* Add error information back into metadata to roll back removal in #394

* Go back to obtaining runtime error info

* re-do codegen too to check that it's all gravy

* Convert DispatchError module errors into a module variant to make them easier to work with

* Fix broken doc link

codegen/src/api/mod.rs

@@ -265,9 +265,8 @@ impl RuntimeGenerator {
                     pallet.calls.as_ref().map(|_| pallet_mod_name)
                 });
 
-        let error_details = errors::generate_error_details(&self.metadata);
-        let error_type = error_details.type_def;
-        let error_fn = error_details.dispatch_error_impl_fn;
+        let has_module_error_impl =
+            errors::generate_has_module_error_impl(&self.metadata, types_mod_ident);
 
         let default_account_data_ident = format_ident!("DefaultAccountData");
         let default_account_data_impl = generate_default_account_data_impl(
@@ -291,12 +290,8 @@ impl RuntimeGenerator {
 
                 /// The default error type returned when there is a runtime issue.
                 pub type DispatchError = #types_mod_ident::sp_runtime::DispatchError;
-
-                // Statically generate error information so that we don't need runtime metadata for it.
-                #error_type
-                impl DispatchError {
-                    #error_fn
-                }
+                // Impl HasModuleError on DispatchError so we can pluck out module error details.
+                #has_module_error_impl
 
                 #default_account_data_impl