Use the new `desired_targets_checked` method exposed by Election Provider

[Ank4n]

Election Provider defines a bound MaxWinners which limits the size of winners it can elect. ElectionProvider also assumes ElectionDataProvider will respect this bound and return a desired_targets() that is lower than this. As such, this call is unsafe to use and caller is expected to handle the case when above assumption fails.

We can improve on this by introducing a desired_targets_checked() method on ElectionProvider which returns an error if the above assumption fails. This is already defined here but unused for now.