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/search-backend-node/src/indexing/BatchSearchEngineIndexer.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { assertError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 import { IndexableDocument } from '@backstage/plugin-search-common';
 import { Writable } from 'node:stream';
 
@@ -66,8 +66,7 @@ export abstract class BatchSearchEngineIndexer extends Writable {
       await this.initialize();
       done();
     } catch (e) {
-      assertError(e);
-      done(e);
+      done(toError(e));
     }
   }
 
@@ -91,8 +90,7 @@ export abstract class BatchSearchEngineIndexer extends Writable {
       this.currentBatch = [];
       done();
     } catch (e) {
-      assertError(e);
-      done(e);
+      done(toError(e));
     }
   }
 
@@ -110,8 +108,7 @@ export abstract class BatchSearchEngineIndexer extends Writable {
       await this.finalize();
       done();
     } catch (e) {
-      assertError(e);
-      done(e);
+      done(toError(e));
     }
   }
 }

plugins/search-backend-node/src/indexing/DecoratorBase.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { assertError } from '@backstage/errors';
+import { toError } from '@backstage/errors';
 import { IndexableDocument } from '@backstage/plugin-search-common';
 import { Transform } from 'node:stream';
 
@@ -60,8 +60,7 @@ export abstract class DecoratorBase extends Transform {
       await this.initialize();
       done();
     } catch (e) {
-      assertError(e);
-      done(e);
+      done(toError(e));
     }
   }
 
@@ -96,8 +95,7 @@ export abstract class DecoratorBase extends Transform {
       this.push(decorated);
       done();
     } catch (e) {
-      assertError(e);
-      done(e);
+      done(toError(e));
     }
   }
 
@@ -110,8 +108,7 @@ export abstract class DecoratorBase extends Transform {
       await this.finalize();
       done();
     } catch (e) {
-      assertError(e);
-      done(e);
+      done(toError(e));
     }
   }
 }