ExceedsStackLimit was meant to be used before running XCMs but it's being used in FrameTransactionalProcessor in instructions

[franciscoaguirre]

This error had no index and was not part of the spec because it was meant to be used only before XCMs were executed, so only locally.
It's being used in the FrameTransactionalProcessor which is called on a lot of instructions.
This means we either need to include it in the spec or change the error to another one that is/will be in the spec.

[ggwpez]

We also match on this variant here:

polkadot-sdk/polkadot/xcm/xcm-builder/src/process_xcm_message.rs

Line 106 in 7ecf3f7

xcm::latest::Error::ExceedsStackLimit => ProcessMessageError::StackLimitReached,

The MQ pallet needs to know that it was a StackExceedsError - or rather: a transient error that may fix itself spontaneously through a user-retry but is not related to weight.