Add error() possibility to app.transaction() callback

From https://community.aseprite.org/t/cancel-a-transaction/24406
and aseprite/aseprite#4946

Author: dacap

api/app.md

@@ -181,15 +181,32 @@ app.transaction(
 ```
 
 Creates a new transaction so you can group several sprite
-modifications in just one undo/redo operation.
+modifications in just one undo/redo operation. If a string is given as
+first argument, that string will be used as the label of this
+undo/redo action (which can be seen in the *Edit > Undo History*
+window).
 
-The function in the argument is called inside the transaction, if the
-function fails, the whole transaction is undone. If the function
-successes, the transaction is committed and then all actions will be
-grouped in just one undo/redo operation.
+The given function is called inside the transaction, if the function
+fails, the whole transaction is undone (i.e. all the steps executed so
+far will be reversed). If the function successes, the transaction is
+committed and then all actions will be grouped in just one undo/redo
+operation.
 
-If a string is given as first argument, that string will be used as a
-label for the undo/redo action.
+You can cancel/reverse/cause an explicit failure of the transaction
+calling the `error()` Lua function, e.g:
+
+```lua
+app.transaction(
+  function()
+    ...
+    if something_is_wrong then
+      error() -- this stops the function and doesn't continue with the
+              -- following lines / the actions so far will be reversed
+              -- automatically leaving the sprite intact
+    end
+    ...
+  end)
+```
 
 ## app.command