v15: Rename error_enum_ty to module_error_enum_ty

[lexnv]

This PR renames the error_enum_ty to better reflect the type details.

Considering that it does not represent the DispatchError emitted by the chain,
it contains only the information needed to decode the ModuleError of the
DispatchError::Module variant.

The ModuleError is represented by a raw form in sp-runtime:

pub struct ModuleError {
    pub index: [u8](https://doc.rust-lang.org/nightly/std/primitive.u8.html),
    pub error: [[u8](https://doc.rust-lang.org/nightly/std/primitive.u8.html); [4](https://doc.rust-lang.org/nightly/std/primitive.array.html)],
    pub message: [Option](https://doc.rust-lang.org/nightly/core/option/enum.Option.html)<&'static [str](https://doc.rust-lang.org/nightly/std/primitive.str.html)>,
}

This added type in the metadata will provide support for better decoding of the module error:

enum ModuleErrorType {
   System(pallet_system::Error),
   ...
}

Needed by: paritytech/substrate#14143

// @paritytech/subxt-team