* Avoid importing k8s.io/kubernetes from pkg/helm

When writing a helm client (e.g. a helm plugin) that talks to tiller importing k8s.io/helm/pkg/helm to get the grpc client is key.
This pkg should not have a dependency to the k8s.io/kubernetes to avoid pulling in a lot of code that is only used within tiller and blow up binary sizes.
Signed-off-by: Fabian Ruff <fabian@progra.de>

* Add references to pull request in errors message
Signed-off-by: Fabian Ruff <fabian@progra.de>

* copy helper function from pkg/storage/driver
Signed-off-by: Fabian Ruff <fabian@progra.de>

* Move storage errors to seperate package
Signed-off-by: Fabian Ruff <fabian@progra.de>

* Keep old  error variables for backward compatibility
Signed-off-by: Fabian Ruff <fabian@progra.de>