frontend-plugin-api: clean up output mismatch error message

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/frontend-plugin-api/src/wiring/createExtension.ts

@@ -148,6 +148,29 @@ export interface LegacyCreateExtensionOptions<
   }): Expand<ExtensionDataValues<TOutput>>;
 }
 
+type ToIntersection<U> = (U extends any ? (k: U) => void : never) extends (
+  k: infer I,
+) => void
+  ? I
+  : never;
+
+type PopUnion<U> = ToIntersection<
+  U extends any ? () => U : never
+> extends () => infer R
+  ? [rest: Exclude<U, R>, next: R]
+  : undefined;
+
+/** @ignore */
+type JoinStringUnion<
+  U,
+  TDiv extends string = ', ',
+  TResult extends string = '',
+> = PopUnion<U> extends [infer IRest extends string, infer INext extends string]
+  ? TResult extends ''
+    ? JoinStringUnion<IRest, TDiv, INext>
+    : JoinStringUnion<IRest, TDiv, `${TResult}${TDiv}${INext}`>
+  : TResult;
+
 /** @ignore */
 export type VerifyExtensionFactoryOutput<
   UDeclaredOutput extends AnyExtensionDataRef,
@@ -162,18 +185,12 @@ export type VerifyExtensionFactoryOutput<
   ? [IRequiredOutputIds] extends [UFactoryOutput['id']]
     ? [UFactoryOutput['id']] extends [UDeclaredOutput['id']]
       ? {}
-      : {
-          'Error: The extension factory has undeclared output(s)': Exclude<
-            UFactoryOutput['id'],
-            UDeclaredOutput['id']
-          >;
-        }
-    : {
-        'Error: The extension factory is missing the following output(s)': Exclude<
-          IRequiredOutputIds,
-          UFactoryOutput['id']
-        >;
-      }
+      : `Error: The extension factory has undeclared output(s): ${JoinStringUnion<
+          Exclude<UFactoryOutput['id'], UDeclaredOutput['id']>
+        >}`
+    : `Error: The extension factory is missing the following output(s): ${JoinStringUnion<
+        Exclude<IRequiredOutputIds, UFactoryOutput['id']>
+      >}`
   : never;
 
 /** @public */