Add with_pays_fee for conversion to DispatchErrorWithPostInfo

[bragov4ik]

Description

Extend WithPostDispatchInfo to allow easier specification of pays_fee when converting to DispatchErrorWithPostInfo.

At the time of PR that added with_weight (#5458), pays_fee field was not present, so it makes sense to add add such function.
For example, instead of

let mut e = DispatchErrorWithPostInfo::from(Error::<T>::InvalidParameters);
e.pays_fee = Pays::No;
e

one can write

Error::<T>::InvalidParameters.with_pays_fee(Pays::No)

My motivation in making this change is inconvenience when working with errors returned from other functions. In particular, I had the following case:

Self::fallible_function().map_err(|e| {
	let mut e = DispatchErrorWithPostInfo::from(e);
	e.pays_fee = Pays::No;
	e
})?;

and this should be cleaner and easier to read:

Self::fallible_function().map_err(|e| e.with_pays_fee(Pays::No))?;

• I haven't found an issue about this and I'm not sure if it should be created, as it's mostly convenience PR.
• Apparently, I don't have permissions to add labels, but I suppose it's something like A0, B0, C1
• It doesn't change existing functions, only adds an extra one. Shouldn't break API, I guess. Correct me if I'm wrong please :)

[cla-bot-2021]

User @bragov4ik, please sign the CLA here.

[bragov4ik]

Also I wanted to implement WithPostDispatchInfo for DispatchErrorWithPostInfo to allow builder pattern for the latter (e.with_pays_fee(Pays::No).with_weight(1234)). However, that impl conflicted with existing <T: Into<DispatchError>> one ("upstream crates may add a new impl of trait" was the error).

One of the ways I see to fix it is to impl for DispatchError instead, but it might break some stuff, so I'm not sure about it :)

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.

[bragov4ik]

Not sure why the CI doesn't pass with (lack of feature in sp-keystore) error. I didn't touch any features 🤔

[KiChjang]

It looks like to me that we really should add a new function called with_post_info, so that callers can freely decide whether or not they want to use the default or modify any of the fields.

These methods should also exist in PostDispatchInfo rather than here as well, as it's not clear that the function actually returns a DispatchErrorWtihPostInfo after calling it.

[juangirini]

Not sure why the CI doesn't pass with (lack of feature in sp-keystore) error. I didn't touch any features 🤔

It seems like the Polkadot companion is off, I will re-run the job.