Remove specialized exception in favor of built-in type

Author: ralexstokes

tests/beacon/test_helpers.py

@@ -63,10 +63,6 @@
 )
 
 
-class UnreachableCodePathError(Exception):
-    pass
-
-
 @pytest.fixture()
 def sample_block(sample_beacon_block_params):
     return SerenityBeaconBlock(**sample_beacon_block_params)
@@ -886,8 +882,7 @@ def _corrupt_vote_count(params):
                 new_vote_count,
             )
     else:
-        msg = "list of ``custody_bit_0_indices`` should not exhaust ``itertools.count``"
-        raise UnreachableCodePathError(msg)
+        raise Exception("Unreachable code path")
 
 
 def _create_slashable_vote_data_messages(params):