Complete error message

[zhanyuanucb]

The previous error message is confusing.
Even though a user passes in kwargs , since allow_new_hparam is set toFalse, the error message will still say that Only hyperparameters named 'kwargs' hyperparameters can contain new entries undefined in default hyperparameters.

[codecov]

Codecov Report

Merging #345 (09a02be) into master (dae9a8f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage   80.43%   80.43%           
=======================================
  Files         135      135           
  Lines       11359    11359           
=======================================
  Hits         9137     9137           
  Misses       2222     2222           

Impacted Files	Coverage Δ
texar/torch/hyperparams.py	86.95% <ø> (ø)

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