Wrong type message in join operation should include variable name

[hadrienk]

in AbstractJoinOperation:

        // Type mismatch check
        List<String> typeMismatches = Lists.newArrayList();
        for (Map.Entry<Component, Map<Dataset, Component>> entry : idMap.entrySet()) {
            Component identifier = entry.getKey();

            Multimap<Class<?>, Dataset> typeMap = ArrayListMultimap.create();
            entry.getValue().entrySet().forEach(datasetComponent -> {
                typeMap.put(
                        datasetComponent.getValue().getType(),
                        datasetComponent.getKey()
                );
            });
            if (typeMap.keySet().size() != 1)
                typeMismatches.add(String.format("%s -> (%s)", identifier, typeMap));
        }
        checkArgument(
                typeMismatches.isEmpty(),
                ERROR_INCOMPATIBLE_TYPES,
                String.join(", ", typeMismatches)
        );