BandwidthTestHandler: More informative error messages
FYI @lugrazet
After my last two MRs, we should get error messages for all the failures we're likely to get. However, currently it is just a single one-size-fits-all message. In fact, there are cases where the comparison fails but the rest of the test is likely to be OK and still useful. This MR improves the handler messaging to distinguish those failures, and also prevents it trying to run the comparison if key information is not present.
Tested locally that we get the correct failure and message if:
- Key info (e.g. total rate) cannot be read,
- index.html is not present,
- Cant get info from ref build