HTTP error handling in client often implicitly expects response to be JSON

[ponyisi]

There are cases like

ServiceX_frontend/servicex/servicex_adapter.py

Line 127 in 7105fd3

o = await r.json()

where we assume we can call r.json() when the server response may not be JSON (this is essentially a certainty for 500 errors for example). The retries will hide this at the moment because they're not picky about the exception type, but if there is a real server error that needs to get propagated to users, this behavior will obfuscate it.

Plan: go through code and fix places where we assume a JSON response in abnormal conditions.