Improve error logs for task manager poller (#197635)

I noticed some scenarios we see error logs from the task poller like
`Failed to poll for work: undefined` making me think `err.message` is
empty in some situations. I'm modifying the code to handle string
situations if ever they occur by performing `err.message || err` and to
also include a stack trace when strings are passed-in.

---------

Co-authored-by: Patrick Mueller <patrick.mueller@elastic.co>

x-pack/plugins/task_manager/server/polling/task_poller.test.ts

@@ -266,6 +266,37 @@ describe('TaskPoller', () => {
     expect(handler.mock.calls[0][0].error.stack).toContain(workError.stack);
   });
 
+  test('still logs errors when they are thrown as strings', async () => {
+    const pollInterval = 100;
+
+    const handler = jest.fn();
+    const workError = 'failed to work';
+    const poller = createTaskPoller<string, string[]>({
+      initialPollInterval: pollInterval,
+      logger: loggingSystemMock.create().get(),
+      pollInterval$: of(pollInterval),
+      pollIntervalDelay$: of(0),
+      work: async (...args) => {
+        throw workError;
+      },
+      getCapacity: () => 5,
+    });
+    poller.events$.subscribe(handler);
+    poller.start();
+
+    clock.tick(pollInterval);
+    await new Promise((resolve) => setImmediate(resolve));
+
+    const expectedError = new PollingError<string>(
+      'Failed to poll for work: failed to work',
+      PollingErrorType.WorkError,
+      none
+    );
+    expect(handler).toHaveBeenCalledWith(asErr(expectedError));
+    expect(handler.mock.calls[0][0].error.type).toEqual(PollingErrorType.WorkError);
+    expect(handler.mock.calls[0][0].error.stack).toBeDefined();
+  });
+
   test('continues polling after work fails', async () => {
     const pollInterval = 100;
 

x-pack/plugins/task_manager/server/polling/task_poller.ts

@@ -151,7 +151,14 @@ export enum PollingErrorType {
 }
 
 function asPollingError<T>(err: Error, type: PollingErrorType, data: Option<T> = none) {
-  return asErr(new PollingError<T>(`Failed to poll for work: ${err.message}`, type, data, err));
+  return asErr(
+    new PollingError<T>(
+      `Failed to poll for work: ${err.message || err}`,
+      type,
+      data,
+      err instanceof Error ? err : new Error(`${err}`)
+    )
+  );
 }
 
 export class PollingError<T> extends Error {