ToolHandle: throw exception if retrieval fails

If a ToolHandle is retrieved with the wrong interface, an StatusCode::FAILURE is returned, but no error message is displayed. This is especially bad when ToolHandles are automatically retrieved by their parent at the end of initialize - initialization will fail with no obvious cause.

This MR addresses this by having ToolHandle::retrieve throw an exception if either the retrieval from the ToolSvc fails, or if the dcast to the required iface fails.

I'm a bit torn about this however. We've gone from returning a StatusCode::FAILURE, which can easily be dealt with at runtime, to throwing an exception, which requires significantly different behaviour from the client. I don't know if there is a real use case for continuing execution if the retrieval fails. I think the best solution would have been to print an error message, and return a FAILURE as before, but there's no easy way to get hold of the MessageSvc from within the ToolHandle.

GaudiExamples/src/AlgTools/MyGaudiAlgorithm.cpp

@@ -49,6 +49,19 @@ StatusCode MyGaudiAlgorithm::initialize()
 
   info() << m_selectedTracks.objKey() << endmsg;
 
+  // m_wrongIfaceTool is being retrieved via the wrong interface.
+  // we expect the retrieve() to throw an exception.
+  try {
+    if ( m_wrongIfaceTool.retrieve().isFailure() ) {
+      error() << "unable to retrieve " << m_wrongIfaceTool.typeAndName() << " (unexpected)" << endmsg;
+      m_wrongIfaceTool.disable();
+    }
+  } catch ( GaudiException& ex ) {
+    info() << "unable to retrieve " << m_wrongIfaceTool.typeAndName() << " (expected) with exception: " << ex.what()
+           << endmsg;
+    m_wrongIfaceTool.disable();
+  }
+
   info() << "....initialization done" << endmsg;
 
   return sc;