Feature: Support HMS/HMT inverters in different countries with different frequency bands

Thanks to @Fribur, @homeautomation2022 and @stefan123t

include/WebApi_errors.h

@@ -15,6 +15,7 @@ enum WebApiError {
     DtuPollZero,
     DtuInvalidPowerLevel,
     DtuInvalidCmtFrequency,
+    DtuInvalidCmtCountry,
 
     ConfigBase = 3000,
     ConfigNotDeleted,
@@ -89,4 +90,4 @@ enum WebApiError {
 
     HardwareBase = 12000,
     HardwarePinMappingLength,
-};
+};