Bug - createResolutionWithExclusion should check if the address is allowed

[stecb]

When we call createResolutionWithExclusion, the excludedContributor address can be whatever address, as we don't do any type of pre-validation to it. This is a problem, as the resolution is still valid, but when we try to approve it, it then breaks.

We should therefore make sure the address for the addressedContributor is one of the possible voters.

[vrde]

I think #132 doesn't fix the problem.

It can happen that at the time of the creation of the resolution with exclusion, the excluded address is a contributor, but at the time of the approval the address is not a contributor anymore.