forwardMigrate2 tries to drop unset index

[erixtekila]

While migrating to version 2 then 3, by using _forwardMigrate() then _forwardMigrate2(), an exception is raised

meteor-roles/roles/roles_server.js

Line 325 in ff9cad1

Roles._dropCollectionIndex(Meteor.users, 'roles._id_1_roles.scope_1')

No such index is present in the user's collection

To Reproduce
Start with a version 1 app.
Add several roles.
Use _forwardMigrate() then _forwardMigrate2() on it.
The migration process stops on this exception

MongoServerError: index not found with name [roles._id_1_roles.scope_1]

Expected behavior
A clean update of the servverside collections.

Environnement

• Meteor 2.12
• OS: OSX
• Browser chrome, firefox

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