fix: drop invalid orders (#105)

# Description
There's one order in a loop of checking for every block. 

Each check, watch tower arrives to the conclusion that the order is
invalid:

<img width="1642" alt="image"
src="https://github.com/cowprotocol/watch-tower/assets/2352112/9e277b91-692d-4194-90ab-4a891c27a29c">

I can see at least 1700 attempts!
<img width="1120" alt="image"
src="https://github.com/cowprotocol/watch-tower/assets/2352112/fd658147-f651-4392-abf3-fe88321638ef">


# Changes
Same as we do in the SDK, we drop the invalid orders so we don't check
again.

Run this in Gnosis Chain and make sure the order is properly dropped.

src/utils/contracts.ts

@@ -18,16 +18,60 @@ const REQUIRED_SELECTORS = [
 
 // Define an enum for the custom error revert hints that can be returned by the ComposableCoW's interfaces.
 export enum CustomErrorSelectors {
+  /**
+   * The owner has not authorized the order
+   */
   PROOF_NOT_AUTHED = "PROOF_NOT_AUTHED",
+
+  /**
+   * The owner has not authorized the order
+   */
   SINGLE_ORDER_NOT_AUTHED = "SINGLE_ORDER_NOT_AUTHED",
+
+  /**
+   * The conditional order didn't pass the swap guard
+   */
   SWAP_GUARD_RESTRICTED = "SWAP_GUARD_RESTRICTED",
+
+  /**
+   * The handler is not supported
+   */
   INVALID_HANDLER = "INVALID_HANDLER",
+
+  /**
+   * The Safe doesn't have the extensible fallback handler set
+   */
   INVALID_FALLBACK_HANDLER = "INVALID_FALLBACK_HANDLER",
+
+  /**
+   * `InterfaceNotSupported` is returned when the contract does not implement the `IERC165` interface
+   */
   INTERFACE_NOT_SUPPORTED = "INTERFACE_NOT_SUPPORTED",
+  /**
+   * `OrderNotValid` is generally returned when elements
+   * of the data struct are invalid. For example, if the `sellAmount` is zero, or the `validTo` is in
+   * the past
+   */
   ORDER_NOT_VALID = "ORDER_NOT_VALID",
+
+  /**
+   * The conditional order has signalled that it should be polled again on the next block
+   */
   POLL_TRY_NEXT_BLOCK = "POLL_TRY_NEXT_BLOCK",
+
+  /**
+   * Stop polling the order
+   */
   POLL_NEVER = "POLL_NEVER",
+
+  /**
+   * The conditional order has signalled that it should be polled again at the given block number
+   */
   POLL_TRY_AT_BLOCK = "POLL_TRY_AT_BLOCK",
+
+  /**
+   * The conditional order has signalled that it should be polled again at the given epoch
+   */
   POLL_TRY_AT_EPOCH = "POLL_TRY_AT_EPOCH",
 }
 
@@ -251,14 +295,13 @@ export function handleOnChainCustomError(params: {
         return dropOrder("The conditional order didn't pass the swap guard");
       // TODO: Add metrics to track this
       case CustomErrorSelectors.ORDER_NOT_VALID:
-      case CustomErrorSelectors.POLL_TRY_NEXT_BLOCK:
-        // OrderNotValid: With the revised custom errors, `OrderNotValid` is generally returned when elements
-        // of the data struct are invalid. For example, if the `sellAmount` is zero, or the `validTo` is in
-        // the past.
-        // PollTryNextBlock: The conditional order has signalled that it should be polled again on the next block.
+        const reason = msgWithSelector(parsedCustomError.message);
         log.info(
-          `Order on safe ${owner} not valid/signalled to try next block.`
+          `Order for ${owner} is invalid. Reason: ${reason}. Deleting order...`
         );
+        return dropOrder(`Invalid order: ${reason}`);
+      case CustomErrorSelectors.POLL_TRY_NEXT_BLOCK:
+        log.info(`Order on safe ${owner} not signalled to try next block`);
         return {
           result: PollResultCode.TRY_NEXT_BLOCK,
           reason: msgWithSelector(parsedCustomError.message),