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-module-migrate/src/lib/versioning/yarn.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { assertError, ForwardedError } from '@backstage/errors';
+import { ForwardedError } from '@backstage/errors';
 import { runOutput } from '@backstage/cli-common';
 
 const versions = new Map<string, Promise<'classic' | 'berry'>>();
@@ -32,7 +32,6 @@ export function detectYarnVersion(dir?: string): Promise<'classic' | 'berry'> {
       });
       return stdout.trim().startsWith('1.') ? 'classic' : 'berry';
     } catch (error) {
-      assertError(error);
       throw new ForwardedError('Failed to determine yarn version', error);
     }
   });