Fail fast on initializing with a different project's databaseUrl

[snarfed]

Originally filed in https://github.com/firebase/firebase-functions/issues/1191 and on StackOverflow.

Hi all! Thanks for Firebase, it's great!

I recently tried to set up a Firebase Cloud Function to trigger on a Realtime Database owned by a different project: the Hacker News API. I passed databaseUrl: https://hacker-news.firebaseio.com/ to admin.initializeApp(), and it deployed fine, but it never triggered.

I eventually learned that Firebase just doesn't support triggering across projects like this. Fair enough. Could you consider failing fast and reporting an error in this case? That would have saved me a fair amount of debugging.

Code below. Thanks in advance!

const functions = require('firebase-functions')
const admin = require('firebase-admin')
admin.initializeApp({databaseUrl: 'https://hacker-news.firebaseio.com/'})

exports.itemUpdated = functions.database.ref('/v0/item/{id}').onCreate((snapshot, context) => {
      functions.logger.log('Item: ', context.params.id, snapshot.val())
      return null
})

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