Validate correct branch in Release object

Avoids throwing exceptions in service and delegates validation to the
domain level

services.py

@@ -8,12 +8,10 @@ class PrepareReleaseService():
         self.release_repo = release_repo
         self.release = release_repo.get_release()
         self.git_api = GitApi()
-        self.main_branch = None        
 
     def __write_and_commit_version(self, version: Version, commit_message: str):
-        if self.main_branch != self.git_api.get_current_branch():
-            raise Exception('Trying to release while not on main branch')
-        
+        self.release.validate(self.release_repo.main_branch)
+
         self.release_repo.version_repository.write_file(version.get_version_string())
         self.git_api.add_file(self.release_repo.version_repository.file)
         self.git_api.commit(commit_message)