Write error messages to stderr and not stdout

DCDBLib/src/virtualsensor.cpp

@@ -219,7 +219,7 @@ int64_t VSensorExpressionImpl::physicalSensorInterpolator(Connection* connection
           readingBefore.value = (int64_t)value;
       }
       else {
-          std::cout << "Cannot find reading for sensor " << sensor.name << " prior to time " << t.getString() << "(" << t.getRaw() << ")" << std::endl;
+          std::cerr << "Cannot find reading for sensor " << sensor.name << " prior to time " << t.getString() << "(" << t.getRaw() << ")" << std::endl;
       }
       cass_iterator_free(rows);
       cass_result_free(cresult);
@@ -265,7 +265,7 @@ int64_t VSensorExpressionImpl::physicalSensorInterpolator(Connection* connection
           readingAfter.value = (int64_t)value;
       }
       else {
-          std::cout << "Cannot find reading for sensor " << sensor.name << " following time " << t.getString() << "(" << t.getRaw() << ")" << std::endl;
+          std::cerr << "Cannot find reading for sensor " << sensor.name << " following time " << t.getString() << "(" << t.getRaw() << ")" << std::endl;
       }
       cass_iterator_free(rows);
       cass_result_free(cresult);