Fix missing error message when slicing with disabled extruder

Following the removal of the get/setMessage methods in the Job class, the StartSliceJob has not been updated and still used them. We now use a specific variable for storing the disabled extruders and properly display them.

CURA-11904

UM/Job.py

@@ -23,7 +23,6 @@ def __init__(self) -> None:
         self._running = False   # type: bool
         self._finished = False  # type: bool
         self._result = None     # type: Any
-        self._message = None    # type: Any
         self._error = None      # type: Optional[Exception]
 
     def run(self) -> None: