Allow disputing a transfer from/to a non-existent state.

Switch requires to results (BadFromIndex & BadToIndex) when confirming
that a sender and reciever exist.
Sync some enums from Types.sol to match up in TS types in client.
Add more extensive test cases for disputeTransitionTransfer for triggering rollbacks.
Typo signautre -> signature.

contracts/libs/Transition.sol

@@ -124,15 +124,14 @@ library Transition {
         uint256 fee,
         Types.StateMerkleProof memory proof
     ) internal pure returns (bytes32 newRoot, Types.Result) {
-        require(
+        bool doesSenderExist =
             MerkleTree.verify(
                 stateRoot,
                 keccak256(proof.state.encode()),
                 senderStateIndex,
                 proof.witness
-            ),
-            "Transition: Sender does not exist"
-        );
+            );
+        if (!doesSenderExist) return (bytes32(0), Types.Result.BadFromIndex);
         (Types.UserState memory newSender, Types.Result result) =
             validateAndApplySender(tokenID, amount, fee, proof.state);
         if (result != Types.Result.Ok) return (bytes32(0), result);
@@ -151,15 +150,14 @@ library Transition {
         uint256 amount,
         Types.StateMerkleProof memory proof
     ) internal pure returns (bytes32 newRoot, Types.Result) {
-        require(
+        bool doesReceiverExist =
             MerkleTree.verify(
                 stateRoot,
                 keccak256(proof.state.encode()),
                 receiverStateIndex,
                 proof.witness
-            ),
-            "Transition: receiver does not exist"
-        );
+            );
+        if (!doesReceiverExist) return (bytes32(0), Types.Result.BadToIndex);
         (Types.UserState memory newReceiver, Types.Result result) =
             validateAndApplyReceiver(tokenID, amount, proof.state);
         if (result != Types.Result.Ok) return (bytes32(0), result);

contracts/libs/Types.sol

@@ -239,6 +239,9 @@ library Types {
         bytes32[] witness;
     }
 
+    /**
+     * @notice Results of a validation check on a transaction or commit
+     */
     enum Result {
         Ok,
         InvalidTokenAmount,
@@ -250,6 +253,8 @@ library Types {
         BadWithdrawRoot,
         BadCompression,
         TooManyTx,
-        BadPrecompileCall
+        BadPrecompileCall,
+        BadFromIndex,
+        BadToIndex
     }
 }