Fix issues with LTI launch while logged in and course creation

[pretendWhale]

Motivation and Context

Two bugs were discovered with lti functionality:

1. when launching LTI from canvas while logged in to MarkUs, users were not properly redirected to the choose_course path
2. When attempting to create a new course, an uncaught error would occur if a course with the same name is already in the markus database.

Your Changes

Description:

1. modified the login method to properly redirect logged-in users.
2. Catch the case when the course already exists and display an error message if the user attempts to create a new course.

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)

Testing

Wrote tests for the changed methods, and tested in the web interface.

Questions and Comments (if applicable)

Checklist

• I have performed a self-review of my own code.
• I have verified that the pre-commit.ci checks have passed.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have added tests for my changes.

Pull request to make documentation changes (if applicable)

[coveralls]

Pull Request Test Coverage Report for Build 7463603247

Warning: This coverage report may be inaccurate.

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A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 91.679%

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Totals
Change from base Build 7226755465:	0.1%
Covered Lines:	38757
Relevant Lines:	41653

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💛 - Coveralls