Modal location aggregation-unit consistency validation masks other validation errors

[jc-harrison]

Describe the bug
ModalLocationSchema validates that all location sub-queries have the same aggregation unit. However, a validation error in one of the sub-queries will result in the ModalLocationSchema validator attempting to access a non-existent 'aggregation_unit' attribute, and consequently raising an AttributeError instead of ValidationError.

This error message is misleading, as the aggregation unit is not the source of the issue. Also, as the error raised is an AttributeError, not ValidationError, this will result in the server failing to respond to FlowAPI, so a FlowClient user would see a generic "Could not get reply for message" 400 error instead of a helpful message with details of the invalid field.

Product
Flowmachine

To Reproduce

>>> params = {
        'query_kind': 'modal_location',
        'locations': [
            {
                'query_kind': 'daily_location',
                'date': '2016-02-10',
                'aggregation_unit': 'admin3',
                'method': 'invalid-method',
            }
        ]
    }

>>> DailyLocationSchema().load(params["locations"][0])
ValidationError: {'method': ['Must be one of: last, most-common.']}

>>> ModalLocationSchema().load(params)
AttributeError: 'list' object has no attribute 'aggregation_unit'

Expected behavior
Attempting to specify an invalid daily location query as input to a modal location should raise a validation error identifying the invalid parameter.

Additional context
ModalLocationSchema is not the only schema that validates aggregation units in this way - other schemas are likely affected by the same issue.