* Added explicit exception raise when no stack found.
Currently, any operation that uses 'get_stack' method from 'CloudFormationBackend' class
will fail with AttributeError or jinja2 exception if ran against non-existing stack(created/deleted)
To fix the issue I explicitly raised a 'ValidationError' exception.
Added tests for boto and boto3 responses.
* Moved non-existing stack tests to 'test_stack_events'
When using 'update_stack' to test raising an exception when the stack doesn't exist
test coverage dropped by 0.5%. I am using stack_events instead.
* Removed some unreachable paths
After adding the exception couple of paths in the code are unreachable as 'get_stack' doesn't return 'None' anymore.
This is the reason why coverall was reporting decreased coverage.
* Removed an unreachable path I missed
* Added couple of tests in cloudformation/models
* Added more assertions around raised exception
* Formatted document using black to fix issue with travis.
8fe5a680a5