qa: improvements to json exception handling in test_webapi
qa: improvements to json exception handling in test_webapi

Use the try/except pattern from test_metrics_enumeration in
all places.  Tweak all to return after reporting an initial
error, to avoid cascading failures observed recently.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp