Handle sp_runtime::ModuleError substrate updates

[lexnv]

The following change was introduced in substrate via Allow pallet error enum variants to contain fields #10242.

DispatchError::Module ( sp_runtime::ModuleError { index: u8, error: u8 } 

->

DispatchError::Module ( sp_runtime::ModuleError { index: u8, error: [u8; 4] } )

This PR handles the error of type [u8; 4] while maintaining backward compatibility.

Closes #491

Testing Done

• new substrate binary

fn module_error_indices(&self) -> Option<::subxt::ModuleErrorRaw> {
    if let Self::Module(module_error) = self {
        Some(::subxt::ModuleErrorRaw {
            pallet_index: module_error.index,
            error: module_error.error,
        })
    } else {
        None
    }
}

• legacy format

fn module_error_indices(&self) -> Option<::subxt::ModuleErrorRaw> {
    if let Self::Module(module_error) = self {
        Some(::subxt::ModuleErrorRaw {
            pallet_index: module_error.index,
            error: [module_error.error, 0, 0, 0],
        })
    } else {
        None
    }
}

• named format

fn module_error_indices(&self) -> Option<::subxt::ModuleErrorRaw> {
    if let &Self::Module { index, error } = self {
        Some(::subxt::ModuleErrorRaw {
            pallet_index: index,
            error: [error, 0, 0, 0],
        })
    } else {
        None
    }
}

[jsdw]

I'm happy for this to merge once @ascjones's comments are addressed :)

[ascjones]

LGTM