Missing information in error message on dumping benchmark records

[jusschwitalla]

If some data are written to metrics (self.metrics.add_metric) which are not json representable an error occurs when the BenchmarkManager tries to write the results.json. For example: "TypeError: Object of type Graph is not JSON serializable"
The problem now is that it can be very hard to find the responsible add_metric call.
It would be very helpfull if the error message would contain the information which attribute it is that cannot be written.