Refactor MergeRequests::BuildService#validate_branches

MergeRequests::BuildService#validate_branches is quite messy right now. When source_branch is not provided (which happens when you just go to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/new) validation procedure will fail, but we will not include any error messages, so frontend will not display them. This is difficult to follow and confusing.

/cc @smcgivern