Add docstring to InvocationCancelledError and expose to new api via exceptions.

Also change from SystemExit error as this is not the corrcect error
to inherit from.

Author: julianstirling

src/labthings_fastapi/dependencies/invocation.py

@@ -33,8 +33,12 @@ def invocation_logger(id: InvocationID) -> logging.Logger:
 InvocationLogger = Annotated[logging.Logger, Depends(invocation_logger)]
 
 
-class InvocationCancelledError(SystemExit):
-    pass
+class InvocationCancelledError(BaseException):
+    """An invocation was cancelled by the user.
+
+    Note that this inherits from BaseException so won't be caught by
+    `except Exception`, it must be handled specifically.
+    """
 
 
 class CancelEvent(threading.Event):

src/labthings_fastapi/exceptions.py

@@ -1,5 +1,7 @@
 """A submodule for custom LabThings-FastAPI Exceptions"""
 
+from .dependencies.invocation import InvocationCancelledError
+
 
 class NotConnectedToServerError(RuntimeError):
     """The Thing is not connected to a server
@@ -9,3 +11,6 @@ class NotConnectedToServerError(RuntimeError):
     connected to a ThingServer. A server connection is needed
     to manage asynchronous behaviour.
     """
+
+
+__all__ = ["NotConnectedToServerError", "InvocationCancelledError"]