px_uploader.py: check for pyserial

If we dont explicitly check for pyserial, we can have the case where the
import works but the Serial object creation fails. However, we don't see
this because we have this huge try/catch block which swallows
everything.

Tools/px_uploader.py

@@ -732,6 +732,16 @@ def main():
         print("WARNING: You should uninstall ModemManager as it conflicts with any non-modem serial device (like Pixhawk)")
         print("==========================================================================================================")
 
+    # We need to check for pyserial because the import itself doesn't
+    # seem to fail, at least not on macOS.
+    try:
+        if serial.__version__:
+            pass
+    except:
+        print("Error: pyserial not installed!")
+        print("    (Install using: sudo pip install pyserial)")
+        sys.exit(1)
+
     # Load the firmware file
     fw = firmware(args.firmware)