Silent failure when `BT_API_AUTH_TOKEN` wasn't provided

[paulirish]

(just breaking this piece of #176 into a diff issue)

Problem

New users are dumb. Like me.

And when I first tried upload-build i hadn't yet set the env variable. The server 401'd (and that was a silentish failure thanks to #176). But it took a minute to realize I hadn't included BT_API_AUTH_TOKEN.

Your CLI docs already include this env var, but I hadn't yet found those docs. :)

Idea

Perhaps a friendly error message could be provided? Something a la..

The server returned a 401 Unauthorized Error. Did you include the correct BT_API_AUTH_TOKEN?

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also wow the docs are really nice. docusaurus seems badass.

[paularmstrong]

I love the idea of including more information in the error responses. Will look into this tonight.