Error if default config is missing *and* command requires config

[cisaacstern]

#88 covers the case of an explicitly passed config that is missing.

It does not fix the case of a missing default config ("pangeo_forge_runner_config.py").

This is by design, because some commands (e.g., expand_meta) don't require config, so we should allow a missing default config to pass through without an error.

But what if a command does require config (e.g., bake) and the default config is missing? Let's resolve that in a separate PR. 🙂

[cisaacstern]

xref #81 which is closely related