wasp: Manager: Avoid printing the stack trace when it can cause problems

On nRF devices if we print with the NUS console disconnected (instead
of never connected) then things we can end up hanging. Better only
to print an exception if the watch class contains a method to do
that.

wasp/wasp.py

@@ -12,7 +12,6 @@ import gc
 import machine
 import watch
 import widgets
-import sys
 
 from apps import *
 
@@ -330,7 +329,9 @@ class Manager():
             except KeyboardInterrupt:
                 raise
             except Exception as e:
-                sys.print_exception(e)
+                # Only print the exception if the watch provides a way to do so!
+                if 'print_exception' in dir(watch):
+                    watch.print_exception(e)
                 self.switch(CrashApp(e))
 
             # Currently there is no code to control how fast the system