Warn about missing environment variables earlier

[kewisch]

I use github token auth. Often I forget to set the token in my environment, with the result that everything is published except for the github version.

It would be nicer if you could break early if the config options are missing, before anything is done. Bonus points for doing the same when using --no-write

[kewisch]

This is better since 111eaf4, I now see >> Error: No username for GitHub release when using --no-write. I haven't yet tested an actual release, but it seems it will still bail after having pushed.