change how IsErrorCode check error with errors.As

[panotza]

This pull request attempts to address an edge case where IsErrorCode receives a wrapped error. For example, the 'ent' package returns an error like this:

if err := c.batchInsert(ctx, tx, insert); err != nil {
    return fmt.Errorf("insert nodes to table %q: %w", c.Nodes[0].Table, err)
}

It is necessary to use errors.As to properly check if the error can be cast to the sqlState interface or not.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a6be916) 77.92% compared to head (2e4e98a) 77.96%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   77.92%   77.96%   +0.04%     
==========================================
  Files          22       22              
  Lines        1327     1330       +3     
==========================================
+ Hits         1034     1037       +3     
  Misses        270      270              
  Partials       23       23              

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