Gracefully fail if Ghost admin not set

This is still ungraceful, in the sense it'll fail spectacularly
later, but at least it gives us the opportunity to run the new
`setup` command first.

Where's that command, you say? Just wait for the next commit! ;-)

functions.js

@@ -13,12 +13,15 @@ const config = require('./config')
 
 const MEDIUM_IMG_CDN = 'https://miro.medium.com/fit/c/'
 
-const ghostAdmin = new GhostAdminAPI({
+try {
+  const ghostAdmin = new GhostAdminAPI({
     url: config.ghost.url,
     version: config.ghost.version,
     key: config.ghost.admin_key,
-})
-
+  })
+} catch(err) {
+  console.error('Your Ghost isn\'t configured. Please run `seance setup` to fix this!')
+}
 /**
  * function [fetchFromMedium]
  * @returns [string] status