The `nonexistant_service_level_exception` can be thrown by service
levels code and propagated up to the CQL server layer, where it is
converted into a CQL protocol error. The aforementioned exception
inherits from `service_level_argument_exception`, which in turn inherits
from `std::invalid_argument` - which doesn't mean much to the CQL layer
and is converted to a generic SERVER_ERROR.
We can do better and return a more meaningful error code for this
exception. Change the base class of service_level_argument_exception to
exceptions::invalid_request_exception which gets converted to an INVALID
error.
The INVALID error code was already being used by the enterprise version,
so this commit just synchronizes error handling with enterprise.
48e7ffc300