errors: add toError utility and migrate assertError usages

Add a `toError` utility function to `@backstage/errors` that converts
unknown values to `ErrorLike` objects. If the value is already error-like
it is returned as-is. Strings are used directly as the error message, and
other values are stringified with a fallback to JSON.stringify to avoid
unhelpful `[object Object]` messages.

Non-error causes passed to `CustomErrorBase` are now converted and stored
using `toError` rather than discarded. Existing `assertError` call sites
across the codebase are migrated to `toError`.

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

plugins/auth-backend/src/identity/FirestoreKeyStore.ts

@@ -23,6 +23,7 @@ import {
   WriteResult,
 } from '@google-cloud/firestore';
 
+import { toError } from '@backstage/errors';
 import { AnyJWK, KeyStore, StoredKey } from './types';
 
 export type FirestoreKeyStoreSettings = Settings & Options;
@@ -66,14 +67,11 @@ export class FirestoreKeyStore implements KeyStore {
     try {
       await keyStore.verify();
     } catch (error) {
+      const err = toError(error);
       if (process.env.NODE_ENV !== 'development') {
-        throw new Error(
-          `Failed to connect to database: ${(error as Error).message}`,
-        );
+        throw new Error(`Failed to connect to database: ${err.message}`);
       }
-      logger?.warn(
-        `Failed to connect to database: ${(error as Error).message}`,
-      );
+      logger?.warn(`Failed to connect to database: ${err.message}`);
     }
   }
 

plugins/auth-backend/src/providers/router.ts

@@ -16,7 +16,7 @@
 
 import { AuthService, LoggerService } from '@backstage/backend-plugin-api';
 import { Config } from '@backstage/config';
-import { assertError, NotFoundError } from '@backstage/errors';
+import { NotFoundError, toError } from '@backstage/errors';
 import {
   AuthOwnershipResolver,
   AuthProviderFactory,
@@ -104,14 +104,17 @@ export function bindProviderRouters(
 
         targetRouter.use(`/${providerId}`, r);
       } catch (e) {
-        assertError(e);
         if (process.env.NODE_ENV !== 'development') {
           throw new Error(
-            `Failed to initialize ${providerId} auth provider, ${e.message}`,
+            `Failed to initialize ${providerId} auth provider, ${
+              toError(e).message
+            }`,
           );
         }
 
-        logger.warn(`Skipping ${providerId} auth provider, ${e.message}`);
+        logger.warn(
+          `Skipping ${providerId} auth provider, ${toError(e).message}`,
+        );
 
         targetRouter.use(`/${providerId}`, () => {
           // If the user added the provider under auth.providers but the clientId and clientSecret etc. were not found.