Reword unknown architecture compiler error

Author: pennam

src/utility/watchdog/Watchdog.cpp

@@ -70,7 +70,7 @@ void watchdog_enable()
     DEBUG_WARNING("%s: watchdog could not be enabled", __FUNCTION__);
   }
 #else
-# error "ARDUINO_ARCH not defined."
+# error "Unknown ARDUINO_ARCH. Supported architectures are ARDUINO_ARCH_SAMD and ARDUINO_ARCH_MBED."
 #endif
 }
 
@@ -85,7 +85,7 @@ void watchdog_reset()
     hal_watchdog_kick();
   }
 #else
-# error "ARDUINO_ARCH not defined."
+# error "Unknown ARDUINO_ARCH. Supported architectures are ARDUINO_ARCH_SAMD and ARDUINO_ARCH_MBED."
 #endif
 }