Should show different error template in case of error

[sypets]

Currently, even if we do see a flash message, the same Templates are displayed as in case of success. This may lead to misleading and ambiguous error messages.

Reproduce

1. Create form
2. In the last tab, answer page, add some confirmation text, e.g. "Thanks for filling out your form. We will process it as soon as possible"
3. fill out email in form incorrectly or make is so that mail to sender and receiver cannot be sent

result after sending form:

More context

Currently, we do not have a good notification and clear messages in case of an error in all cases:

1. Due to some bugs, flash message is not shown
2. Even if mail to receiver fails, mail to sender is sent as if everything is ok
3. Even if flash message is shown, the normal output is displayed as if everthing is ok (see this issue)