fix _raise_deferred_exc() (pantsbuild#7008)

### Problem

The `_raise_deferred_exc(self)` method in `daemon_pants_runner.py` hasn't ever been tested. As a result, it causes an easily fixable error: the issue can be easily reproduced if you register an option twice in the same task and then run with pantsd enabled (you get the wrong exception, because `exc_type` isn't needed to construct the exception again, that's what `exc_value` is for).

### Solution

- Appropriately destructure `sys.exc_info()` (if that was what was used to populate `self._deferred_exception`) and re-raise the exception with its original traceback.

### Result

This error is fixed, but not yet tested -- see pantsbuild#7220.

src/python/pants/bin/daemon_pants_runner.py

@@ -231,18 +231,16 @@ def nailgunned_stdio(cls, sock, env, handle_stdin=True):
       ) as finalizer:
         yield finalizer
 
+  # TODO: there's no testing for this method, and this caused a user-visible failure -- see #7008!
   def _raise_deferred_exc(self):
     """Raises deferred exceptions from the daemon's synchronous path in the post-fork client."""
     if self._deferred_exception:
-      exc_type, exc_value, exc_traceback = self._deferred_exception
-      if exc_type == GracefulTerminationException:
-        self._exiter.exit(exc_value.exit_code)
       try:
-        # Expect `_deferred_exception` to be a 3-item tuple of the values returned by sys.exc_info().
-        # This permits use the 3-arg form of the `raise` statement to preserve the original traceback.
-        raise_with_traceback(exc_type(exc_value), exc_traceback)
-      except ValueError:
-        # If `_deferred_exception` isn't a 3-item tuple, treat it like a bare exception.
+        exc_type, exc_value, exc_traceback = self._deferred_exception
+        raise_with_traceback(exc_value, exc_traceback)
+      except TypeError:
+        # If `_deferred_exception` isn't a 3-item tuple (raising a TypeError on the above
+        # destructuring), treat it like a bare exception.
         raise self._deferred_exception
 
   def _maybe_get_client_start_time_from_env(self, env):
@@ -327,6 +325,10 @@ def post_fork_child(self):
         runner.run()
       except KeyboardInterrupt:
         self._exiter.exit_and_fail('Interrupted by user.\n')
+      except GracefulTerminationException as e:
+        ExceptionSink.log_exception(
+          'Encountered graceful termination exception {}; exiting'.format(e))
+        self._exiter.exit(e.exit_code)
       except Exception:
         ExceptionSink._log_unhandled_exception_and_exit()
       else: