Add runtime typechecks for screen.find etc. #351

Short overview
Methods like find and similar should perform runtime type checks for their correct parameter types.

Use case
Improved DX

Detailed description
Currently a wrong type passed to screen.find instead of a proper Image will result in a cryptic OpenCV error message.
This is not very helpful to debug the actual root cause, so these methods should perform runtime type checks to ensure users are passing an Image