Tools: use Python 3, improve import checks

In more detail:
- Change shebang to Python 3.
- Suggest installation using pip3 as user.

Tools/mavlink_shell.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 Open a shell over MAVLink.
@@ -11,16 +11,27 @@ from __future__ import print_function
 import sys, select
 import termios
 from timeit import default_timer as timer
+from argparse import ArgumentParser
 
 try:
     from pymavlink import mavutil
-    import serial
-except:
-    print("Failed to import pymavlink.")
-    print("You may need to install it with 'pip install pymavlink pyserial'")
+except ImportError as e:
+    print("Failed to import pymavlink: " + e)
     print("")
-    raise
-from argparse import ArgumentParser
+    print("You may need to install it with:")
+    print("    pip3 install --user pymavlink")
+    print("")
+    sys.exit(1)
+
+try:
+    import serial
+except ImportError as e:
+    print("Failed to import pyserial: " + e)
+    print("")
+    print("You may need to install it with:")
+    print("    pip3 install --user pyserial")
+    print("")
+    sys.exit(1)
 
 
 class MavlinkSerialPort():