Update test to new error message in go 1.16

[jawn-smith]

As of go 1.16, the error message in text/template for this specific case has changed.

[jawn-smith]

This latest commit addresses issue #259 by checking for a substring that is present in both go 1.15 and go 1.16.

[codecov]

Codecov Report

Merging #260 (2c2ba6f) into main (482713a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #260   +/-   ##
=======================================
  Coverage   82.68%   82.68%           
=======================================
  Files          14       14           
  Lines        1178     1178           
=======================================
  Hits          974      974           
  Misses        141      141           
  Partials       63       63           

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[nicksnyder]

Thanks!