Prevent TypeError on mispelled collections

[jankapunkt]

Related to #43

The code is now first attempting to retrieve the underlying Mongo.Collection without directly accessing the filesCollection reference:

const mongoCollection = Mongo.Collection.get
  ? Mongo.Collection.get(this.data.atts.collection)
  : Meteor.connection._mongo_livedata_collections[this.data.atts.collection]

If the mongoCollection is present the filesCollection reference will be assigned to this.collection otherwise a lookup in the global scope is performed.

this.collection = mongoCollection
  ? mongoCollection.filesCollection
  : global[this.data.atts.collection]

In worst case this.collection is null, which will causes the already implemented 404 Error on line 46 to throw:

if (!this.collection) {
  throw new Meteor.Error(404, '[meteor-autoform-files] No such collection "' + this.data.atts.collection + '"');
}

This should give developers a more accurate information on the issue than the previous TypeError

[dr-dimitru]

@jankapunkt thank you a lot for this PR. 👍