Protect against env vars not being set

[colearendt]

Related to #11

If the proper env vars are not set, we should have apps, Rmds, etc. exit with a friendly error message to be clear that more configuration is necessary, because otherwise the proper actions can be a bit difficult for users to grok from the error messages.

[slopp]

I wonder if this could be a feature of the markdown or config package; it seems like this conditional error message is helpful, and silly that we are copying the same code everywhere.

[colearendt]

I like that idea a lot. required_env_vars as a YAML config or something... at simplest, we could always just write a helper function that does it for you.