improved meaningfulness of error message for column_to_rownames()

[alexwhan]

The error message from column_to_rownames() currently refers to num2 instead of the var argument value.

This PR removes num2 and adds the var argument value to the error message

[codecov]

Codecov Report

Merging #399 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #399   +/-   ##
=======================================
  Coverage   90.39%   90.39%           
=======================================
  Files          23       23           
  Lines        1083     1083           
=======================================
  Hits          979      979           
  Misses        104      104

Impacted Files	Coverage Δ
R/rownames.R	100% <100%> (ø)	⬆️

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[krlmlr]

Thanks! (Although the original implementation seems to have passed all relevant tests :-o )