Added explicit exception raise when no stack found. (#3559)

* 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.

tests/test_cloudformation/test_cloudformation_stack_crud.py

@@ -570,6 +570,15 @@ def test_describe_stack_events_shows_create_update_and_delete():
 
     list(stack_events_to_look_for).should.be.empty
 
+    with pytest.raises(BotoServerError) as exp:
+        conn.describe_stack_events("non_existing_stack")
+    err = exp.value
+    err.message.should.equal("Stack with id non_existing_stack does not exist")
+    err.body.should.match(r"Stack with id non_existing_stack does not exist")
+    err.error_code.should.equal("ValidationError")
+    err.reason.should.equal("Bad Request")
+    err.status.should.equal(400)
+
 
 @mock_cloudformation_deprecated
 def test_create_stack_lambda_and_dynamodb():