Improve message of the expression parser if type information is missing.

• Suggest to provide missing type information to property of the top level algorithm sequence, in case no type information is available for a certain variable.
• Distinguish between missing type information and missing inheritance information.

PhysicsAnalysis/CommonTools/ExpressionEvaluation/Root/MultipleProxyLoader.cxx

@@ -14,6 +14,7 @@
 
 #include <stdexcept>
 #include <iostream>
+#include <sstream>
 
 namespace ExpressionParsing {
   MultipleProxyLoader::~MultipleProxyLoader()
@@ -53,7 +54,16 @@ namespace ExpressionParsing {
       break;
     }
     if (!foundProxyLoader) {
-      throw std::runtime_error("MultipleProxyLoader: unable to find valid proxy loader for "+varname);
+      std::stringstream msg;
+      msg << "MultipleProxyLoader: unable to find valid proxy loader for " << varname << "."
+          << " If it is an xAOD element or container which is read from the input file"
+          << " this problem may occur if it is not accessed anywhere in the job via read handles."
+          << " The problem can be mitigated by providing the missing information in the property "
+          << " ExtraDataForDynamicConsumers of the sequence which has the property "
+          << " ProcessDynamicDataDependencies set to True."
+          << " The property takes a list of strings of the form \'type/container-name\' e.g."
+          << " \'xAOD::TrackParticleContainer/InDetTrackParticles\'.";
+      throw std::runtime_error(msg.str());
     }
     return result;
   }