Prevent ill formed set --pre causing scheduler failures.

cylc/flow/task_pool.py

@@ -1715,17 +1715,25 @@ def _standardise_prereqs(self, prereqs: 'List[str]') -> 'List[Tokens]':
                 msg = self.config.get_taskdef(
                     pre['task']
                 ).outputs[output][0]
+                cycle=standardise_point_string(pre['cycle'])
             except KeyError:
                 # The task does not have this output.
                 LOG.warning(
                     f"output {pre.relative_id_with_selectors} not found")
                 continue
-            _prereqs.append(
-                pre.duplicate(
-                    task_sel=msg,
-                    cycle=standardise_point_string(pre['cycle'])
+            except WorkflowConfigError as exc:
+                # The workflow does not have the task from --pre:
+                LOG.warning(f'Invalid pre task name set:\n    {exc.args[0]}')
+            except PointParsingError as exc:
+                # The CP from --pre is invalid:
+                LOG.warning(f'Invalid pre cycle point set:\n    {exc.args[0]}')
+            else:
+                _prereqs.append(
+                    pre.duplicate(
+                        task_sel=msg,
+                        cycle=cycle,
+                    )
                 )
-            )
         return _prereqs
 
     def _standardise_outputs(

tests/integration/test_task_pool.py

@@ -1383,6 +1383,40 @@ async def test_set_prereqs(
         assert qux.state.prerequisites_all_satisfied()
 
 
+async def test_set_bad_prereqs(
+    flow,
+    scheduler,
+    start,
+    log_filter,
+):
+    """Check manual setting of prerequisites.
+
+    """
+    id_ = flow({
+        'scheduler': {
+            'allow implicit tasks': 'True',
+            'cycle point format': '%Y'},
+        'scheduling': {
+            'initial cycle point': '2040',
+            'graph': {'R1': "foo => bar"}},
+    })
+    schd = scheduler(id_)
+
+    def set_prereqs(prereqs):
+        """Shorthand so only varible under test given as arg"""
+        schd.pool.set_prereqs_and_outputs(
+            ["2040/bar"], None, prereqs, ['all'])
+
+    async with start(schd) as log:
+        # Invalid - task name wildcard:
+        set_prereqs(["2040/*"])
+        assert 'Illegal task name' in log.messages[-1]
+
+        # Invalid cycle point wildcard.
+        set_prereqs(["*/foo"])
+        assert 'Invalid ISO 8601' in log.messages[-1]
+
+
 async def test_set_outputs_live(
     flow,
     scheduler,