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/scaffolder-backend/src/scaffolder/tasks/TaskWorker.ts

@@ -16,7 +16,7 @@
 
 import { AuditorService, LoggerService } from '@backstage/backend-plugin-api';
 import type { MetricsService } from '@backstage/backend-plugin-api/alpha';
-import { assertError, InputError, stringifyError } from '@backstage/errors';
+import { InputError, stringifyError, toError } from '@backstage/errors';
 import { ScmIntegrations } from '@backstage/integration';
 import { PermissionEvaluator } from '@backstage/plugin-permission-common';
 import {
@@ -228,12 +228,12 @@ export class TaskWorker {
       await task.complete('completed', { output });
       await auditorEvent?.success();
     } catch (error) {
-      assertError(error);
+      const err = toError(error);
       await auditorEvent?.fail({
-        error,
+        error: err,
       });
       await task.complete('failed', {
-        error: { name: error.name, message: error.message },
+        error: { name: err.name, message: err.message },
       });
     }
   }

plugins/scaffolder-backend/src/service/helpers.ts

@@ -27,7 +27,7 @@ import {
   stringifyEntityRef,
 } from '@backstage/catalog-model';
 import { Config } from '@backstage/config';
-import { assertError, InputError, NotFoundError } from '@backstage/errors';
+import { InputError, NotFoundError, toError } from '@backstage/errors';
 import { CatalogService } from '@backstage/plugin-catalog-node';
 import { TemplateEntityV1beta3 } from '@backstage/plugin-scaffolder-common';
 import fs from 'fs-extra';
@@ -47,13 +47,13 @@ export async function getWorkingDirectory(
     await fs.access(workingDirectory, fs.constants.F_OK | fs.constants.W_OK);
     logger.info(`using working directory: ${workingDirectory}`);
   } catch (err) {
-    assertError(err);
+    const error = toError(err);
     logger.error(
       `working directory ${workingDirectory} ${
-        err.code === 'ENOENT' ? 'does not exist' : 'is not writable'
+        error.code === 'ENOENT' ? 'does not exist' : 'is not writable'
       }`,
     );
-    throw err;
+    throw error;
   }
   return workingDirectory;
 }