Don't flash the device unless we successfully detected the device

Nicer error messages are nicer.

bin/kaleidoscope-builder

@@ -212,6 +212,15 @@ clean () {
 }
 
 reset_device () {
+    if [ -z $DEVICE_PORT ]; then
+        echo "Couldn't autodetect the keyboard's serial port."
+        echo "If you see this message and your keyboard is connected to your computer,"
+        echo "it probably means that our serial port detection logic is buggy or incomplete."
+        echo
+        echo "Please report this issue at https://github.com/keyboardio/Kaleidoscope";
+        exit 0;
+    fi
+
     ${RESET_DEVICE}
 }