Fix HTTP Status Code returned for a Failed Precondition error

[cjcormack]

Fixes #972 (and reverses #657). This now returns the semantically correct 400 response, rather than a 412. This matches the gRPC documentation.

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[codecov-io]

Codecov Report

Merging #974 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #974   +/-   ##
=======================================
  Coverage   53.25%   53.25%           
=======================================
  Files          40       40           
  Lines        4007     4007           
=======================================
  Hits         2134     2134           
  Misses       1675     1675           
  Partials      198      198

Impacted Files	Coverage Δ
runtime/errors.go	47.36% <100%> (ø)	⬆️

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[cjcormack]

Have signed the CLA for my other email address.

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CLAs look good, thanks!

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