Fix to fail gracefully if current version missing

[zeroasterisk]

Use Case:

Given developer has a branch with a new Migration version.
When the developer changes branch back to master/integration.
Then the Migration should not throw an Error, crashing Meteor.

• Change _findIndexByVersion() to return false if not found.
• Change test for index into _migrateTo(), start=graceful fail, end=error.
• Add a new test to verify functionality.

[zol]

@zeroasterisk Thanks for submitting this PR. I hear your use case but I'm not sure I'm comfortable with solving it like this. The reason being that I don't like the control record being inconsistent with the migrations that have been defined. This could point to a more serious issue that becomes effectively masked with just a warning. I realize this is a pain in development right now.

[zenweasel]

Just a note that this is causing an issue with production deployments in the rare case where we need to rollback to the previous version. You get an error saying it can't find the migration (because it existing in the version you are rolling back from) and the app completely doesn't stop.

Since rolling back a production version is not that common of an occurrence it would be better if it didn't just cause the app to crash.