Add missing error to PriceFeedMock

[0xAkuti]

Adds missing error definition for the error LZ_PriceFeed_NotAnOPStack which is used in PriceFeedMock._getL1LookupId() and currently causing issues (LayerZero-Labs#978).

Error: 
Compiler run failed:
Error (7576): Undeclared identifier.
   --> lib/devtools/packages/test-devtools-evm-foundry/contracts/mocks/PriceFeedMock.sol:184:16:
    |
184 |         revert LZ_PriceFeed_NotAnOPStack(l2Eid);
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^

The alternative would be to add it in ILayerZeroPriceFeed where the error LZ_PriceFeed_UnknownL2Eid which was previously used is defined, but since LZ_PriceFeed_NotAnOPStack seems to only be used in this mock and not in layerzero v2 adding it here seems to be the way to go.