better error messages for policy schema failures

[mathause]

Since the policy schema is pretty simple, I was thinking about extracting information from the error message: if the error message is

Invalid policy definition: [] is not of type 'object'

Failed validating 'type' in schema['properties']['policy']['properties']['overrides']:
    {'additionalProperties': False,
     'patternProperties': {'^[a-z][-a-z_]*': {'format': 'date',
                                              'type': 'string'}},
     'type': 'object'}

On instance['policy']['overrides']:
    []

Then we have the path of the wrong object in On instance['policy']['overrides'], and the actual and expected types in the first line. This should be sufficient to create a new error message that is friendlier to a python audience.

Originally posted by @keewis in #32 (comment)