Ignore `greenback` import error if not installed

tractor/_debug.py

@@ -801,6 +801,7 @@ def _set_trace(
 # python's builtin breakpoint() hook to this runtime aware version.
 def pause_from_sync() -> None:
     print("ENTER SYNC PAUSE")
+    try:
         import greenback
         __tracebackhide__ = True
 
@@ -817,6 +818,8 @@ def pause_from_sync() -> None:
                 # release_lock_signal=task_can_release_tty_lock,
             ))
         )
+    except ModuleNotFoundError:
+        log.warning('NO GREENBACK FOUND')
 
     db, undo_sigint = mk_mpdb()
     Lock.local_task_in_debug = 'sync'