Show Warning/Error if the operator has a different API version

[DragonDev1906]

I've initially used the version 0.1.3 on npm, since npm didn't have a newer version and there was no newer release on github either. It would have been nice if the SDK complained/warned me that it is not compatible with the API provided by the Erdstall operator instead of silently failing in the deposit/subscribe calls. This could prevent some issues when an outdated SDK version is used.