refactor(onboarding): warning when onboarding pr cant be found (#15724)

lib/workers/repository/onboarding/pr/index.spec.ts

+import type { RequestError, Response } from 'got';
 import {
   RenovateConfig,
   defaultConfig,
@@ -203,5 +204,38 @@ describe('workers/repository/onboarding/pr/index', () => {
         'DRY-RUN: Would create onboarding PR'
       );
     });
+
+    describe('ensureOnboardingPr() throws', () => {
+      const response = partial<Response>({ statusCode: 422 });
+      const err = partial<RequestError>({ response });
+
+      beforeEach(() => {
+        jest.resetAllMocks();
+        GlobalConfig.reset();
+        git.deleteBranch.mockResolvedValue();
+      });
+
+      it('throws when trying to create a new PR', async () => {
+        platform.createPr.mockRejectedValueOnce(err);
+        await expect(
+          ensureOnboardingPr(config, packageFiles, branches)
+        ).toReject();
+        expect(git.deleteBranch).toHaveBeenCalledTimes(0);
+      });
+
+      it('deletes branch when PR already exists but cannot find it', async () => {
+        err.response.body = {
+          errors: [{ message: 'A pull request already exists' }],
+        };
+        platform.createPr.mockRejectedValueOnce(err);
+        await expect(
+          ensureOnboardingPr(config, packageFiles, branches)
+        ).toResolve();
+        expect(logger.warn).toHaveBeenCalledWith(
+          'Onboarding PR already exists but cannot find it. It was probably created by a different user.'
+        );
+        expect(git.deleteBranch).toHaveBeenCalledTimes(1);
+      });
+    });
   });
 });

lib/workers/repository/onboarding/pr/index.ts

@@ -152,14 +152,16 @@ If you need any further assistance then you can also [request help here](${confi
       logger.info({ pr: pr.displayNumber }, 'Onboarding PR created');
       await addParticipants(config, pr);
     }
-  } catch (err) /* istanbul ignore next */ {
+  } catch (err) {
     if (
-      err.statusCode === 422 &&
+      err.response?.statusCode === 422 &&
       err.response?.body?.errors?.[0]?.message?.startsWith(
         'A pull request already exists'
       )
     ) {
-      logger.debug('Onboarding PR already exists but cannot find it');
+      logger.warn(
+        'Onboarding PR already exists but cannot find it. It was probably created by a different user.'
+      );
       await deleteBranch(config.onboardingBranch);
       return;
     }