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/devtools-backend/src/api/DevToolsBackendApi.ts

@@ -32,7 +32,7 @@ import os from 'node:os';
 import fs from 'fs-extra';
 import { Lockfile } from '../util/Lockfile';
 import { memoize } from 'lodash';
-import { assertError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 import { LoggerService } from '@backstage/backend-plugin-api';
 
 /** @public */
@@ -179,7 +179,7 @@ export class DevToolsBackendApi {
       const data = ConfigReader.fromConfigs(sanitizedConfigs).get();
       configInfo.config = data;
     } catch (error) {
-      assertError(error);
+      const err = toError(error);
       // The config is not valid for some reason but we want to be able to see it still
       const config = {
         data: this.config.get() as JsonObject,
@@ -194,10 +194,10 @@ export class DevToolsBackendApi {
       const data = ConfigReader.fromConfigs(sanitizedConfigs).get();
       configInfo.config = data;
       configInfo.error = {
-        name: error.name,
-        message: error.message,
-        messages: error.messages as string[] | undefined,
-        stack: error.stack,
+        name: err.name,
+        message: err.message,
+        messages: err.messages as string[] | undefined,
+        stack: err.stack,
       };
     }