Better error reporting in Elector class.

                    distribute = getattr(self, func)

        # Get the destination hypervisor candidates:

        candidates = self._get_candidates(self.FILTERS[mtype])

        candidates, errors = self._get_candidates(self.FILTERS[mtype])

        # Check if VM migration election raised an error:

        if errors:

            # If errors are found, we report only the first of them, because we

            # have currently no way to report problem for each vm:

            vm, desc = errors[0]

            raise ElectionError('No destination found for %r, last destination'

                                ' filtered by %s' % (vm['id'], desc))

        # Distributes VMs to each candidate:

        migration_plan = distribute(mtype, candidates)

        hvs = self._server.list(self._query_dest, show=hv_tags)

        candidates = []

        errors = []

        # Filters the candidates:

        for vm in vms:

            vm_dest = copy(hvs)

            for func, desc in filterfuncs:

                vm_dest = func(vm, vm_dest)

                # VM is added to errors if none destination HV is found for it:

                if not vm_dest:

                    errors.append((vm, desc))

                    break

            else:

                candidates.append((vm, vm_dest))

        return candidates

        return candidates, errors

#####

##### Distribution algorithm methods: