Get RateTestHandler ready for prmon, exception handling

Work towards lhcb-dpa/project#199.

Once the rate tests are wrapped by prmon this handler can read and parse output from prmon: prmon.txt and prmon.json.

A preview of the updated webpage is here.

The handler will also work even if prmon is not ready yet.

Add an exception handling so that logs can be uploaded anyway, this is needed to investigate the error here.