Gracefully handle unexpected output from version check

The [method to return the latest `tabris` version](https://github.com/eclipsesource/generator-tabris-js/blob/a9f07baf456835750baba2e9e464ec3753a2f6a4/app/generator.js#L208) sometimes fails for unknown reasons. This causes the rest of the generator to use `undefined` as the latest version and soon causes a premature end of the script. I've seen a couple reports of this happening but haven't pinpointed a cause. In any case, an accurate error message should be shown or a warning could be printed with the script deferring to some other source (e.g. the installed version of the cli, as the cli is often released in lockstep with the js framework).