[Minor] Negative values in count and page provoke a 500 error

[fra967]

If count is set to a negative number, it is passed down to MongoDB provoking a "bad skip value in query" exception and a 500 error is returned by API. A similar situation happens with page <= 0.
The error is recorded in API log.

Queries with invalid count and page values should either be corrected with default values, or captured and logged as invalid, but not executed in MongoDB.