Stop logging expected GraphQL errors in test output

[jamescrowley]

The current test set up intercepts GraphQL errors to print them as output. This is useful, as they warn about missing mock data and other errors. However, these messages are mixed up in 'expected' failures that we've set up in the tests themselves.

This PR switches to separate classes for our fake errors so we can exclude them from the console output.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.85%. Comparing base (52e1022) to head (9b1046a).
Report is 83 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1535   +/-   ##
=======================================
  Coverage   85.85%   85.85%           
=======================================
  Files         229      229           
  Lines       21709    21709           
  Branches     1927     1927           
=======================================
  Hits        18639    18639           
  Misses       3034     3034           
  Partials       36       36           

Flag	Coverage Δ
backend	99.07% <ø> (ø)
frontend	82.68% <ø> (ø)

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[jamescrowley]

2.0: FE test/linting improvements from James