fix(log): unify error messages for vim.ui_attach/decor providers #33005

Problem:  Error messages that cause a vim.ui_attach() namespace to
          detach are not visible in the message history. Decoration
          provider and vim.ui_attach error messages are dissimilar.
Solution: Emit vim.ui_attach() errors as an actual message in addition
          to logging it. Adjust error message format.

src/nvim/lua/executor.h

@@ -44,6 +44,9 @@ typedef enum {
   kRetLuaref,  ///< return value becomes a single Luaref, regardless of type (except NIL)
 } LuaRetMode;
 
+/// Maximum number of errors in vim.ui_attach() and decor provider callbacks.
+enum { CB_MAX_ERROR = 3, };
+
 /// To use with kRetNilBool for quick truthiness check
 #define LUARET_TRUTHY(res) ((res).type == kObjectTypeBoolean && (res).data.boolean == true)