Add error property to worker.py (#2865)

worker.error is described in the worker guide documentation but the property is not present in the code.

Author: mardanst

src/textual/worker.py

@@ -244,6 +244,11 @@ def result(self) -> ResultType | None:
         """The result of the worker, or `None` if there is no result."""
         return self._result
 
+    @property
+    def error(self) -> BaseException | None:
+        """The exception raised by the worker, or `None` if there was no error."""
+        return self._error
+
     def update(
         self, completed_steps: int | None = None, total_steps: int | None = -1
     ) -> None: