Add task outcome and outcome exception.

Author: tobiasraabe

src/_pytask/outcomes.py

@@ -103,6 +103,7 @@ class TaskOutcome(Enum):
     SKIP = auto()
     SKIP_PREVIOUS_FAILED = auto()
     FAIL = auto()
+    WOULD_BE_EXECUTED = auto()
 
     @property
     def symbol(self) -> str:
@@ -218,6 +219,10 @@ class Persisted(PytaskOutcome):
     """Outcome if task should persist."""
 
 
+class WouldBeExecuted(PytaskOutcome):
+    """Outcome if a task would be executed."""
+
+
 class Exit(Exception):
     """Raised for immediate program exits (no tracebacks/summaries)."""