Improve RequiredEventNotFoundError message

Signed-off-by: Christophe Bedard <bedard.christophe@gmail.com>

tracetools_analysis/tracetools_analysis/processor/__init__.py

@@ -282,7 +282,7 @@ class Processor():
     """Processor class, which dispatches events to event handlers."""
 
     class RequiredEventNotFoundError(RuntimeError):
-        """When a trace does not contain one event required by an EventHandler."""
+        """When a trace does not contain at least one event required by an EventHandler."""
 
         pass
 
@@ -380,11 +380,14 @@ class Processor():
     ) -> None:
         event_names = self.get_event_names(events)
         # Check names separately so that we can know which event from which handler is missing
+        missing_events: Dict[str, Set[str]] = defaultdict(set)
         for handler in self._expanded_handlers:
             for name in handler.required_events():
                 if name not in event_names:
+                    missing_events[handler.__class__.__name__].add(name)
+        if missing_events:
             raise self.RequiredEventNotFoundError(
-                        f'missing event {name} for {handler.__class__.__name__}'
+                f'missing events: {dict(missing_events)}'
             )
 
     def process(