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/auth/src/components/ConsentPage/useConsentSession.ts

@@ -23,7 +23,7 @@ import {
 import { useCallback } from 'react';
 import useAsync from 'react-use/esm/useAsync';
 import useAsyncFn from 'react-use/esm/useAsyncFn';
-import { isError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 
 interface Session {
   id: string;
@@ -111,9 +111,7 @@ export const useConsentSession = (opts: { sessionId?: string }) => {
     if (sessionState.error) {
       return {
         status: 'error',
-        error: isError(sessionState.error)
-          ? sessionState.error.message
-          : 'Failed to load consent request',
+        error: toError(sessionState.error).message,
       };
     }
     if (sessionState.value) {
@@ -133,7 +131,7 @@ export const useConsentSession = (opts: { sessionId?: string }) => {
           await handleActionInternal(action, state.session);
         } catch (err) {
           alertApi.post({
-            message: isError(err) ? err.message : `Failed to ${action} consent`,
+            message: toError(err).message,
             severity: 'error',
           });
         }