Require config.file/resource/url to exist regardless of parse options

These are overriding what was set programmatically, so even if
the app wants to allow the config file to be missing normally,
since these were manually specified they should have to exist.
People get confused when they set a nonexistent file and it
silently doesn't work.

Fixes #97
Merge pull request #129 from typesafehub/wip/havocp-config-load-nonexistent

Require config.file/resource/url to exist regardless of parse options