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-common/src/run.ts

@@ -17,7 +17,7 @@
 import { ChildProcess, SpawnOptions } from 'node:child_process';
 import spawn from 'cross-spawn';
 import { ExitCodeError } from './errors';
-import { assertError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 
 /**
  * Callback function that can be used to receive stdout or stderr data from a child process.
@@ -193,14 +193,14 @@ export async function runOutput(
 
     return Buffer.concat(stdoutChunks).toString().trim();
   } catch (error) {
-    assertError(error);
+    const err = toError(error);
 
-    (error as Error & { stdout?: string }).stdout =
+    (err as Error & { stdout?: string }).stdout =
       Buffer.concat(stdoutChunks).toString();
-    (error as Error & { stderr?: string }).stderr =
+    (err as Error & { stderr?: string }).stderr =
       Buffer.concat(stderrChunks).toString();
 
-    throw error;
+    throw err;
   }
 }