fix: print exception to stderr on worker boot failure

When a worker fails to boot, the exception is now printed to stderr
(in addition to being logged), consistent with AppImportError handling.
This makes boot failures more visible to users.

Closes #2933

gunicorn/arbiter.py

@@ -631,8 +631,10 @@ class Arbiter:
             print("%s" % e, file=sys.stderr)
             sys.stderr.flush()
             sys.exit(self.APP_LOAD_ERROR)
-        except Exception:
+        except Exception as e:
             self.log.exception("Exception in worker process")
+            print("%s" % e, file=sys.stderr)
+            sys.stderr.flush()
             if not worker.booted:
                 sys.exit(self.WORKER_BOOT_ERROR)
             sys.exit(-1)