If the Meteor tooling can't generate a bundle, we should provide a clear error message

[paulproteus]

Right now it seemingly hangs forever and the wrench says it can't find main.js.

[paulproteus]

One reason this can happen is if the app has iOS or Android platforms enabled, but Sandstorm's Meteor tooling doesn't have the capacity to build those.

[paulproteus]

For now, a workaround if users run into this is:

meteor remove-platform ios meteor remove-platform android

[zarvox]

Note to self: in the event we want to patch or suggest the user adjust this, the platform list is stored in .meteor/platforms