revise error message for better upgrading

[haifeng-jin]

Revised the error message for returning None in Tuner.run_trial(). It is now
more specific. It is easier for the user to upgrade their code to adapt to the
latest KerasTuner.

[codecov-commenter]

Codecov Report

Base: 95.10% // Head: 95.10% // No change to project coverage 👍

Coverage data is based on head (d08d064) compared to base (2525396).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #776   +/-   ##
=======================================
  Coverage   95.10%   95.10%           
=======================================
  Files          46       46           
  Lines        3087     3087           
=======================================
  Hits         2936     2936           
  Misses        151      151           

Impacted Files	Coverage Δ
keras_tuner/engine/tuner_utils.py	89.03% <ø> (ø)

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