Add additional debugging output

[jftanner]

This PR adds some debugging output for when context.config returns an empty object (or null). This resulted from #566, where I had neglected to set GHE_HOST so Octokit called public Github instead of GHE. Hopefully this debugging helps future users identify the same situation.

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[travi]

is there documentation that would be worth linking to in this log output that would have given hints toward why the config was unable to be read?

[jftanner]

That's a good question. This is the relevant documentation: https://probot.github.io/docs/configuration/

Though, maybe I should add an additional check that verifies that the URLs in the webhook payload match GHE_HOST variable. That'd be a known failure state with a known fix, whereas there might be other reasons why context.config might fail.

[jftanner]

I've added that change. Unfortunately, I can't test it properly because I'm stuck on an unrelated problem with ESM.