Introduce an exception specific to handling an unknown node ID

See https://github.com/Textualize/textual/pull/1535#discussion_r1065873649

src/textual/widgets/_tree.py

@@ -513,6 +513,9 @@ class Tree(Generic[TreeDataType], ScrollView, can_focus=True):
         else:
             return line.node
 
+    class UnknownID(Exception):
+        """Exception raised when referring to an unknown `TreeNode` ID."""
+
     def get_node_by_id(self, node_id: NodeID) -> TreeNode[TreeDataType]:
         """Get a tree node by its ID.
 
@@ -521,8 +524,14 @@ class Tree(Generic[TreeDataType], ScrollView, can_focus=True):
 
         Returns:
             TreeNode[TreeDataType]: The node associated with that ID.
+
+        Raises:
+            Tree.UnknownID: Raised if the `TreeNode` ID is unknown.
         """
-        return self._nodes[node_id]
+        try:
+            return self._nodes[node_id]
+        except KeyError:
+            raise self.UnknownID(f"Unknown TreeNode ID: {node_id}") from None
 
     def validate_cursor_line(self, value: int) -> int:
         """Prevent cursor line from going outside of range."""

tests/tree/test_tree_get_node_by_id.py

@@ -12,5 +12,5 @@ def test_get_tree_node_by_id() -> None:
     assert tree.get_node_by_id(tree.root.id).id == tree.root.id
     assert tree.get_node_by_id(child.id).id == child.id
     assert tree.get_node_by_id(grandchild.id).id == grandchild.id
-    with pytest.raises(KeyError):
+    with pytest.raises(Tree.UnknownID):
         tree.get_node_by_id(cast(NodeID, grandchild.id + 1000))