Implement HW's exception-handling suggestion

scripts/phase0/build_spec.py

@@ -6,6 +6,7 @@ def build_phase0_spec(sourcefile, outfile):
     code_lines = []
     code_lines.append("""
 
+import copy
 from typing import (
     Any,
     Callable,

specs/core/0_beacon-chain.md

@@ -1262,16 +1262,21 @@ def get_genesis_beacon_state(genesis_validator_deposits: List[Deposit],
 
 ## Beacon chain state transition function
 
-The post-state corresponding to a pre-state `state` and a block `block` is defined as `state_transition(state, block)`. State transitions that trigger an unhandled exception (e.g. a failed `assert` or an out-of-range list access) are considered invalid.
+The post-state corresponding to a pre-state `state` and a block `block` is defined as `state_transition(state, block)`.
 
 ```python
 def state_transition(state: BeaconState, block: BeaconBlock) -> BeaconState:
+    pre_state = copy.deepcopy(state)
+    try:
         # Process slots (including those with no blocks) since block
         process_slots(state, block.slot)
         # Process block
         process_block(state, block)
-    # Return post-state
+    except Exception:
+        # State is not advanced on exceptions (e.g. a failed `assert` or an out-of-range list access)
+        return pre_state
     return state
+
 ```
 
 ```python