return error of type 'mssql.Error' directly, because the Number in 'm…

[sdghchj]

…ssql.Error' is useful for logic behavior.

fix the issue: #577

[codecov[bot]]

Codecov Report

Merging #578 into master will not change coverage. The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #578   +/-   ##
=======================================
  Coverage   68.72%   68.72%           
=======================================
  Files          23       23           
  Lines        5190     5190           
=======================================
  Hits         3567     3567           
  Misses       1410     1410           
  Partials      213      213           

Impacted Files	Coverage Δ
tds.go	61.03% <50.00%> (ø)

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[denisenkom]

Can you add tests for this new feature. Also how do you make sure it is backward compatible?