[frontend/courses] registration to an already registered course makes the app crash

[anthonygego]

Describe the bug
If the course registration page is accessed when it is not relevant (no registration possible or user already registered), an Exception is raised.

The issue comes from these lines :

INGInious/inginious/frontend/pages/course_register.py

Lines 29 to 30 in fcb729e

	if self.user_manager.course_is_user_registered(course, username) or not course.is_registration_possible(user_info):
	return redirect(self.app.get_homepath() + "/course/" + course.get_id())

as the method should actually returns a tuple. It should date from the web.py to flask migration when redirect were made with raise statement, while flask actually treat them as typical Response objects.

INGInious installation details

• Version v0.8+

To Reproduce
Steps to reproduce the behavior:

1. Copy the course registration link : /register/<course_id>
2. Register to the course.
3. Access the registration link once registered.

Expected behavior
The initial redirection or any other feedback message.

Reported by: @Donaschmi