refactor: contract integration test should streamline the error for better testing

[chungquantin]

cc: @Daanvdplas

fn decoded<T: Decode>(result: ExecReturnValue) -> T {
	match <T>::decode(&mut &result.data[2..]) {
		Ok(value) => value,
		Err(_) => panic!("\nTest failed by trying to decode `{:?}` into `T`\n", result),
	}
}

At the moment, any method failed will end up at line 13:39 which is inside the decoded method. This is very hard to debug. My recommendation is to refactor the decoded method to return Result<T> so it can streamline the error back to the main function. Something like:

fn decoded<T: Decode>(result: ExecReturnValue) -> Result<T> {
	<T>::decode(&mut &result.data[2..]).map_err("\nTest failed by trying to decode `{:?}` into `T`\n", result)
}

[Daanvdplas]

Yes this was something that could be improved, great suggestion!

[Daanvdplas]

Fixed by #130