fix: deleted courses do not break program details page

Sometimes learners have certificates in old course runs which've been
deleted. When this happens loading the learner's program progress can
result in 500 errors. This corrects those by choosing to count any
non-existent course the learner has certificates for as counting
towards program completion, effectively assuming that availability
dates have passed for all such courses.

Also fixes an error with a condition related to how we determine
whether an enrolled course is considered "in progress". The previous
version of the code had a bug that would result in a lot more courses
being marked "in progress" for the purpose of program completion than
actually were.

JIRA:EDUCATOR-5787