fill3: Handle unhandled exceptions.

- Make sure all exceptions are handled. Exceptions occuring in
  update_screen and on_terminal_input are no longer lost.
  the program stops and the traceback is shown.
- Tried to use set_exception_handler, but no go.

fill3/fill3/__init__.py

@@ -409,9 +409,24 @@ class Fixed:
 ##########################
 
 
+EXCEPTION = None
 _LAST_APPEARANCE = []
 
 
+def handle_exception():
+    def decorating_func(func):
+        def wrapper(*args):
+            try:
+                return func(*args)
+            except Exception as exc:
+                global EXCEPTION
+                EXCEPTION = exc
+                SHUTDOWN_EVENT.set()
+        return wrapper
+    return decorating_func
+
+
+@handle_exception()
 def draw_screen(widget):
     global _LAST_APPEARANCE
     appearance = widget.appearance(os.get_terminal_size())
@@ -419,6 +434,7 @@ def draw_screen(widget):
     _LAST_APPEARANCE = appearance
 
 
+@handle_exception()
 def patch_screen(widget):
     global _LAST_APPEARANCE
     appearance = widget.appearance(os.get_terminal_size())
@@ -437,6 +453,7 @@ async def update_screen(screen_widget):
         APPEARANCE_CHANGED_EVENT.clear()
 
 
+@handle_exception()
 def on_terminal_input(screen_widget):
     term_code = sys.stdin.read()
     if term_code.startswith(terminal.MOUSE):
@@ -474,7 +491,8 @@ async def tui(title, screen_widget):
                 loop.remove_reader(sys.stdin)
         finally:
             update_task.cancel()
-
+    if EXCEPTION is not None:
+        raise EXCEPTION
 
 ##########################
 
@@ -490,6 +508,8 @@ class _Screen:
     def on_keyboard_input(self, term_code):
         if term_code in ["q", terminal.ESC, terminal.CTRL_C]:
             SHUTDOWN_EVENT.set()
+        elif term_code == "e":
+            raise AssertionError  # Program should shutdown and show exception.
         else:
             self.content = Filler(Text(repr(term_code)))
             APPEARANCE_CHANGED_EVENT.set()