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/cli-node/src/git/GitUtils.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { assertError, ForwardedError } from '@backstage/errors';
+import { ForwardedError, toError } from '@backstage/errors';
 import { targetPaths } from '@backstage/cli-common';
 import { runOutput } from '@backstage/cli-common';
 
@@ -28,13 +28,10 @@ export async function runGit(...args: string[]) {
     });
     return stdout.trim().split(/\r\n|\r|\n/);
   } catch (error) {
-    assertError(error);
-    if (
-      'code' in error &&
-      typeof (error as { code?: number }).code === 'number'
-    ) {
-      const code = (error as { code?: number }).code;
-      const stderr = (error as { stderr?: string }).stderr;
+    const err = toError(error);
+    if ('code' in err && typeof (err as { code?: number }).code === 'number') {
+      const code = (err as { code?: number }).code;
+      const stderr = (err as { stderr?: string }).stderr;
       const msg = stderr?.trim() ?? `with exit code ${code}`;
       throw new Error(`git ${args[0]} failed, ${msg}`);
     }

packages/cli-node/src/pacman/yarn/Yarn.ts

@@ -14,11 +14,7 @@
  * limitations under the License.
  */
 
-import {
-  assertError,
-  ForwardedError,
-  NotImplementedError,
-} from '@backstage/errors';
+import { ForwardedError, NotImplementedError } from '@backstage/errors';
 import { PackageInfo, PackageManager } from '../PackageManager';
 import { Lockfile } from '../Lockfile';
 import { YarnVersion } from './types';
@@ -96,7 +92,6 @@ function detectYarnVersion(dir?: string): Promise<YarnVersion> {
         : 'berry';
       return { version: versionString, codename };
     } catch (error) {
-      assertError(error);
       throw new ForwardedError('Failed to determine yarn version', error);
     }
   });