gdb/python: update events test to handle missing exit_code

The test gdb.python/py-events.exp sets up a handler for the gdb.exited
event.  Unfortunately the handler is slightly broken, it assumes that
the exit_code attribute will always be present.  This is not always
the case.

In a later commit I am going to add more tests to py-events.exp test
script, and in so doing I expose the bug in our handling of gdb.exited
events.

Just to be clear, GDB itself is fine, it is the test that is not
written correctly according to the Python Events API.

So, in this commit I fix the Python code in the test, and extend the
test case to exercise more paths through the Python code.

Additionally, I noticed that the gdb.exited event is used as an
example in the documentation for how to write an event handler.
Unfortunately the same bug that we had in our test was also present in
the example code in the manual.

So I've fixed that too.

After this commit there is no functional change to GDB.

gdb/doc/python.texi

@@ -3202,7 +3202,10 @@ Here is an example:
 @smallexample
 def exit_handler (event):
     print ("event type: exit")
-    print ("exit code: %d" % (event.exit_code))
+    if hasattr (event, 'exit_code'):
+        print ("exit code: %d" % (event.exit_code))
+    else:
+        print ("exit code not available")
 
 gdb.events.exited.connect (exit_handler)
 @end smallexample