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

packages/config-loader/src/schema/collect.ts

@@ -24,7 +24,7 @@ import {
 } from 'node:path';
 import { ConfigSchemaPackageEntry } from './types';
 import { JsonObject } from '@backstage/types';
-import { assertError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 
 type Item = {
   name?: string;
@@ -249,9 +249,9 @@ async function compileTsSchemas(
         );
       }
     } catch (error) {
-      assertError(error);
-      if (error.message !== 'type Config not found') {
-        throw error;
+      const err = toError(error);
+      if (err.message !== 'type Config not found') {
+        throw err;
       }
     }
 

packages/config-loader/src/sources/EnvConfigSource.ts

@@ -15,7 +15,7 @@
  */
 
 import { AppConfig } from '@backstage/config';
-import { assertError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 import { JsonObject } from '@backstage/types';
 import { AsyncConfigSourceGenerator, ConfigSource } from './types';
 
@@ -165,7 +165,6 @@ function safeJsonParse(str: string): [Error | null, any] {
   try {
     return [null, JSON.parse(str)];
   } catch (err) {
-    assertError(err);
-    return [err, str];
+    return [toError(err), str];
   }
 }

packages/config-loader/src/sources/transform/apply.ts

@@ -15,7 +15,7 @@
  */
 
 import { JsonObject, JsonValue } from '@backstage/types';
-import { assertError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 import { TransformContext, TransformFunc } from './types';
 import { isObject } from './utils';
 import { createSubstitutionTransform } from './substitution';
@@ -50,8 +50,7 @@ export async function applyConfigTransforms(
           break;
         }
       } catch (error) {
-        assertError(error);
-        throw new Error(`error at ${path}, ${error.message}`);
+        throw new Error(`error at ${path}, ${toError(error).message}`);
       }
     }