Search a Customer by customer properties

This method allows checking if a Customer with certain attributes (e.g. email or phone number) already exists in the Cloud4Wi account.
* Check if customer with provided via CustomerQuery properties already exists
* @param query - <CustomerQuery> object with at least one field initialized
* @param onSuccess - invoked if `customer exists` call was proceeded without exceptions
* @param onError - invoked if exception occurred
- (void) checkIfCustomerExists: (CustomerQuery *) query
onSuccess: (void (^)(BOOL)) onSuccess
onError: (void (^)(NSError *error)) onError;