Store log errors in state directory

[AGWA]

Instead of writing log errors to stderr, write them to a file in the state directory. When reporting a health check failure, include the path to the file and the last several lines.

See #104 for background.

[schildbach]

To keep things simple, I think they could be kept in a looping buffer (of size: several) in memory as well. If we're talking about the HTTP errors, maybe per CT log. Storing them separately on disk will need truncating sooner or later.

[AGWA]

There will be one error file per CT log. The health check seems like a logical time to rotate/truncate the file.