prepare for using STDERR as an error channel

We'll spit out any YAML parsing errors to STDERR so that the output
channel remains clean and completely YAML
filter documents that have no document separator

/^---/ is more of a "document header" than anything. If there isn't any
document header, and no --- at the end, we should still filter the yaml.
prevent "undefined" warnings

If the definedness of each side of a comparison isn't equal, the
comparison cannot be equal.

I just now realized that this is completely wrong and will not prevent
all the definedness warnings...