Improve error message for measures field names

.. when trying to find the right values for the measures() datafilter
   function e.g. measures("Body", "weight") it was not clear why this
   was failing. Added more information to the error message.

src/Core/DataFilter.cpp

@@ -1996,7 +1996,8 @@ void Leaf::validateFilter(Context *context, DataFilterRuntime *df, Leaf *leaf)
                             int field = context->athlete->measures->getFieldSymbols(group).indexOf(field_symbol);
                             if (field < 0 && field_symbol != "date") {
                                 leaf->inerror = true;
-                                DataFiltererrors << QString(tr("invalid measures field '%1' for group '%2'.").arg(field_symbol).arg(group_symbol));
+                                DataFiltererrors << QString(tr("invalid measures field '%1' for group '%2', should be one of: %3.").arg(field_symbol).arg(group_symbol)
+                                .arg(context->athlete->measures->getFieldSymbols(group).join(", ")));
                             }
                         }
                     }