Major semver update and refactor

semver/exceptions.py

@@ -0,0 +1,18 @@
+class SemverException(Exception):
+    """Semver base exception"""
+    pass
+
+
+class NoMergeFoundException(SemverException):
+    """No merge found in commit message"""
+    pass
+
+
+class NotMainBranchException(SemverException):
+    """Not on main branch"""
+    pass
+
+
+class NoGitFlowException(SemverException):
+    """No git flow branch found"""
+    pass