Improve error handling in ArcanistRuntime when failing to load libraries

Summary: Ref T13490. This fixes one bug in D21025 (loading relative to "arcanist/" rather than the parent directory) and improves behavior when a library fails to load (we prompt the user to continue).

Test Plan:

• Ran arc list with bad library specifications, got a sensible error and an option to continue.
• Ran arc list with a library specification that existed next to "arcanist/", got a library load instead of an error.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21029