Validation errors returned via flowapi are difficult to debug

[maxalbert]

When parameter validation of query parameters fails (and also if there is some internal error in flowmachine), an error message will be propagated up through flowapi. However, currently this often doesn't contain enough information to debug the issue without setting debugging breakpoints inside flowmachine and intercepting the execution.

Here is an example error message received by flowclient:

flowclient.client.FlowclientConnectionError: Something went wrong. API returned with status code 400. Error message: 'Parameter validation failed.'. Payload: {'0': {'locations': {'0': {'aggregation_unit': ['Must be one of: admin0, admin1, admin2, admin3.']}}, 'metric': {'0': {'reference_location': {'0': {'aggregation_unit': ['Must be one of: admin0, admin1, admin2, admin3.']}}}}}}

It would be good if this mentioned the input parameters for which validation fails, and also printed the dictionaries in a more readable fashion.