Make email_from and subject required attributes of the email_content schema.

Update the format_checkers to raise the specific exception that why the validator can handle multiple messages.
Which led to a refactor of build_error_message.

app/v2/notifications/notification_schemas.py

@@ -80,7 +80,7 @@ email_content = {
         "body": {"type": "string"},
         "subject": {"type": "string"}
     },
-    "required": ["body"]
+    "required": ["body", "from_email", "subject"]
 }
 
 post_email_response = {