override str method for custom exception

Author: sufyankhanrao

apimatic_core/exceptions/signature_verification_error.py

@@ -9,3 +9,6 @@ class SignatureVerificationError(Exception):
     def __init__(self, message):
         self.message = message
         super().__init__(self.message)
+
+    def __str__(self):
+        return self.message