Fix incorrect error message when using a platform image without storage_service_name parameter

[minamijoyo]

Hi there,

Derived from the following question of Stack Overflow: https://stackoverflow.com/questions/44947178/create-ubuntu-server-resource-on-azure-with-terraform-io

* azure_instance.web: When using a platform image, the 'storage' parameter is required

I think an error message when using a platform image without storage_service_name parameter looks strange and probably wrong.

Fixing the error message itself is easy, so I fixed it, but actually I'm not an Azure user 😅 So, please check this fix is doing the right thing 😇