Exit debug (#2554)

* show single error by default

* changelog

* show numbers of errors

* changelog

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 - App `title` and `sub_title` attributes can be set to any type https://github.com/Textualize/textual/issues/2521
+- Only a single error will be written by default, unless in dev mode ("debug" in App.features) https://github.com/Textualize/textual/issues/2480
 - Using `Widget.move_child` where the target and the child being moved are the same is now a no-op https://github.com/Textualize/textual/issues/1743
 - Calling `dismiss` on a screen that is not at the top of the stack now raises an exception https://github.com/Textualize/textual/issues/2575
 - `MessagePump.call_after_refresh` and `MessagePump.call_later` will not return `False` if the callback could not be scheduled. https://github.com/Textualize/textual/pull/2584

src/textual/app.py

@@ -1630,8 +1630,22 @@ class App(Generic[ReturnType], DOMNode):
 
     def _print_error_renderables(self) -> None:
         """Print and clear exit renderables."""
+        error_count = len(self._exit_renderables)
+        if "debug" in self.features:
             for renderable in self._exit_renderables:
                 self.error_console.print(renderable)
+            if error_count > 1:
+                self.error_console.print(
+                    f"\n[b]NOTE:[/b] {error_count} errors show above.", markup=True
+                )
+        elif self._exit_renderables:
+            self.error_console.print(self._exit_renderables[0])
+            if error_count > 1:
+                self.error_console.print(
+                    f"\n[b]NOTE:[/b] 1 of {error_count} errors show. Run with [b]--dev[/] to see all errors.",
+                    markup=True,
+                )
+
         self._exit_renderables.clear()
 
     async def _process_messages(