[payload] Set status code on error rather than query status

[john-bodley]

@mistercrunch previously you used to raise an exception if there was no-data, which would then get caught here and thus the non-200 status code would ensure that the error message was reported.

When I removed the exception, and instead set the error message (to ensure consistency with how other query errors were handled) the exception wasn't raised, and the response status code was defined purely on whether the query failed or not rather than also checking the presence of an error.

Note in the future I'll try to take a pass at unifying some of this logic; standardizing payloads, determining when exceptions should/shouldn't be thrown, etc.

[michellethomas]

lgtm

[codecov-io]

Codecov Report

Merging #4567 into master will not change coverage.
The diff coverage is 50%.

@@          Coverage Diff           @@
##           master   #4567   +/-   ##
======================================
  Coverage    71.2%   71.2%           
======================================
  Files         187     187           
  Lines       14785   14785           
  Branches     1083    1083           
======================================
  Hits        10527   10527           
  Misses       4255    4255           
  Partials        3       3

Impacted Files	Coverage Δ
superset/views/core.py	71.15% <50%> (ø)	⬆️

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