Refactored CanvasProvideCourseSite methods. Replaced nested if-then conditionals with ordered return statements (or exceptions) which halt the execution of the task.
Exceptions are used when non-expected situations arise, with exception handling in CanvasCourseProvisionController to ensure delivery of error message to front-end stack (AngularJS).

Expanded Rspec tests for CanvasProvideCourseSite, and CanvasCourseProvisionController
Removed unnecessary whitespace
Updated logger error to include term slug in #filter_courses_by_ccns
Merge pull request #1750 from redconfetti/CLC-2633-refactor-squashed

Refactored CanvasProvideCourseSite methods.