mavproxy.py: correct printing of shlex exception

[peterbarker]

message is not an attribute of e (any more?)

Before:

MANUAL>   File "/home/pbarker/.local/lib/python3.10/site-packages/MAVProxy-1.8.70-py3.10.egg/EGG-INFO/scripts/mavproxy.py", line 756, in process_stdin
MANUAL> AttributeError: 'ValueError' object has no attribute 'message'
MANUAL> Unknown command 'File "/home/pbarker/.local/lib/python3.10/site-packages/MAVProxy-1.8.70-py3.10.egg/EGG-INFO/scripts/mavproxy.py", line 756, in process_stdin'
Unknown command 'AttributeError: 'ValueError' object has no attribute 'message''

after:

MANUAL> Caught shlex exception: No closing quotation