Fix, test, and document behavior for configuration loading

- [x] What happens if at startup, one rule (out of several) in one domain (out of several) can't be parsed?
- [x] Same, but during config reloading, not during startup.
- [x] What happens when configuration contains duplicate domains?
- [x] What happens when a domain contains duplicate rules?
- [x] What happens with YAML files containing [several documents](https://gettaurus.org/docs/YAMLTutorial/#YAML-Multi-Documents)?
- [x] What is the minimal accepted configuration file? Is it the same for Fencer and for lyft/ratelimit?

Also:

- [x] (done in #46, not documented) Turns out that lyft/ratelimit loads all files, not just `.yml` files.
- [x] (done in #46, not documented) And it loads them recursively.
- [x] Would it look at `.foo/bar.yaml` if `RUNTIME_IGNOREDOTFILES` is enabled?
- [x] `RUNTIME_IGNOREDOTFILES` in general is not tested by our testsuite by our testsuite, but should be.

Also:

- [x] When lyft/ratelimit finds no configs in the dir, it responds to requests with OK.
- [x] When lyft/ratelimit doesn't find the dir symlink, it responds to requests with OK.
- [x] (partly done in #35, not tested properly, not documented) When lyft/ratelimit finds the config dir but even one file is corrupted (broken YAML or correct YAML but broken field names), it responds to requests with ERROR.
- [x] When lyft/ratelimit can't read a file (e.g. `chmod 0`-ed), other files are loaded correctly and it responds to requests with OK.

And more:

- [ ] Q: What happens to symlinks to files outside the config dir?
A: I checked and they are loaded just fine.
- [ ] Q: Symlinks to directories outside the config dir?
A: They are not followed.
- [ ] Q: Symlink cycles?
A: They are detected ("Too many levels of symbolic links") and other files are loaded correctly.