Improve debuggability

[xlc]

I had a hard time to debug the XCM weight issue caused by #6785. All the error message I got is XcmError::Barrier.

I understand why propagate error is hard but we need an alternative.

One way is to add enough logs to indicate why an error is occurred. I think we already drop logging in production build so it shouldn't have any overhead. And then in releases, it will be great to also make a production build with logging and we can use wasm override to use those runtime to debug errors.

For example, we could refactor this code to log weight limit and max weight and say it failed due to under weighted

polkadot/xcm/xcm-builder/src/barriers.rs

Lines 95 to 105 in f1a45b3

	BuyExecution { weight_limit: Limited(ref mut weight), .. }
	if weight.all_gte(max_weight) =>
	{
	*weight = max_weight;
	Ok(())
	},
	BuyExecution { ref mut weight_limit, .. } if weight_limit == &Unlimited => {
	*weight_limit = Limited(max_weight);
	Ok(())
	},
	_ => Err(()),

[bkchr]

Good idea!

Cc @KiChjang

[KiChjang]

In #6670, I'm adding a RejectReason to all barriers so that you know exactly why a particular XCM didn't pass the barrier. I could split out the PR so that we get the RejectReason logic in, and leave the blocking barrier implementation for later.

[KiChjang]

Should be fixed by #7098.

[xlc]

There are other areas needs improvement.
e.g. I am debugging why I am getting MaxWeightInvalid and no idea what's the required weight

polkadot/xcm/xcm-executor/src/lib.rs

Line 554 in 0c9143e

ensure!(weight.all_lte(require_weight_at_most), XcmError::MaxWeightInvalid);