gio/cancellable-future: Use the same wording of Gio for errors

It would be nicer to just return the actual glib::Error, but that would
imply breaking the API sadly, so let's just craft it manually

Author: 3v1n0

gio/src/cancellable_future.rs

@@ -171,15 +171,15 @@ where
 
 impl From<Cancelled> for glib::Error {
     fn from(_: Cancelled) -> Self {
-        glib::Error::new(IOErrorEnum::Cancelled, "Task cancelled")
+        glib::Error::new(IOErrorEnum::Cancelled, "Operation was cancelled")
     }
 }
 
 impl std::error::Error for Cancelled {}
 
 impl Debug for Cancelled {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "Task cancelled")
+        write!(f, "Operation was cancelled")
     }
 }