Invalid runtime config file paths don't raise any error

[bitprophet]

What it says on the tin. I was writing some tests around #556 and typoed the runtime config file name; got no error about it whatsoever. Feels bad.

[weisjohn]

confirmed; also, passing the --debug flag doesn't indicate this error either.

[Mukller]

I've opened PR #1080 with a minimal fix.

Root cause: Config._load_file() catches IOError with errno == 2 (ENOENT) and silently logs a debug message regardless of whether the path came from an implicit search or an explicit --config/runtime_path= argument. The implicit search is supposed to silently skip missing files, but the explicit runtime path should fail loudly.

Fix in three changes:

1. invoke/exceptions.py — new ConfigFileNotFound(IOError) exception class with a .path attribute.
2. invoke/config.py — in the errno == 2 handler, raise ConfigFileNotFound(str(filepath)) from e when absolute=True (runtime config). All implicit tiers keep their silent-skip behaviour.
3. tests/config.py — two regression tests verifying the exception is raised and that .path matches the supplied path.

ConfigFileNotFound extends IOError so existing except IOError handlers in user code are unaffected.