text: Title: Improve import error message when the client is imported before generation
  Body: ## Problem
  
  <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
  
  Currently it may not be immediately clear to users that they have to run `prisma generate` when they encounter this error:
  
  ```
  >>> from prisma import Prisma
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ImportError: cannot import name 'Prisma' from 'prisma'
  ```
  
  ## Suggested solution
  
  <!-- A clear and concise description of what you want to happen. -->
  
  Python supports module level `__getattr__`, we could probably make use of this to provide an improved error message.
  