Don't crash application if invoked too early

[Cykelero]

When the host application attempts to send a telemetry event, but hasn't yet initialized the TelemetryManager, the manager calls fatalError():

public static var shared: TelemetryManager {
    if let telemetryManager = initializedTelemetryManager {
    /* […] */
    } else {
        fatalError("Please call TelemetryManager.initialize(...) before accessing the shared telemetryManager instance.")
    }

}

As a result, if in a released app, a rarely occurring code path unexpectedly calls TelemetryManager.send() too early, the app will crash.
This is not good! A failure to send a telemetry event is not a fatal event, and the SDK should not make this crashing choice on the behalf of the host app. In my opinion, an assertionFailure call would be much more appropriate.

[Cykelero]

Fixed by #146.