Gracefully handle moved issues

[str4d]

TERMINATE_AT takes a set of issues, maps them to (repo_id, issue_number) tuples, and then later expects them to be in the DAG. This starts failing if an issue gets moved, e.g. zcash/librustzcash#1349 moved to zcash/wallet#5.

Rather than the renderer blowing up, we should check that all issues in TERMINATE_AT exist, and return a nicer error before trying to build the graph.