changed value error output in attempt to fix issue #14.

[gwenwhite]

As suggested by Jenny in PR #24, I replayed the changes in commit 2701ce4 onto the current master branch with git cherry-pick as seen below:

git checkout master 
git pull upstream master
git checkout -b value-error-2
git cherry-pick 2701ce4

Hoping this will be a way of getting around some issues I was facing with the other pull request

[codecov]

Codecov Report

Merging #26 (5aa85e9) into master (adb1764) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files           5        5           
  Lines         224      224           
=======================================
  Hits          223      223           
  Misses          1        1           

Impacted Files	Coverage Δ
cmeutils/gsd_utils.py	100.00% <100.00%> (ø)

[jennyfothergill]

Nice work! 🙌