Use name of wind_farm in error raising only if not None

Author: SabineHaas

windpowerlib/wind_farm.py

@@ -218,7 +218,8 @@ def assign_power_curve(self, wake_losses_model='power_efficiency_curve',
                                  "each wind turbine needs a power curve " +
                                  "but `power_curve` of wind turbine " +
                                  "{} is {}.".format(
-                                     item['wind_turbine'].name,
+                                     item['wind_turbine'].name if
+                                     item['wind_turbine'].name else '',
                                      item['wind_turbine'].power_curve))
         # Initialize data frame for power curve values
         df = pd.DataFrame()
@@ -246,8 +247,8 @@ def assign_power_curve(self, wake_losses_model='power_efficiency_curve',
                         "`efficiency` is needed if " +
                         "`wake_losses_model´ is '{0}', but ".format(
                             wake_losses_model) +
-                        "`efficiency` of {0} is {1}.".format(
-                            self.name, self.efficiency))
+                        "`efficiency` of wind farm {0} is {1}.".format(
+                            self.name if self.name else '', self.efficiency))
             # Get original power curve
             power_curve = pd.DataFrame(
                 turbine_type_dict['wind_turbine'].power_curve)