implement error equality

[ymohl-cl]

Fixes ##487

Description

// EqError returns a matcher that matches on equality message error (only one an error interface type)
//
// Example usage:
//   var err1 error = errors.New("content error 1")
//   var err2 error = errors.New("content error 2")
//   var errCustom1 error = CustomError.Error{msg: "content error 1"}
//   var errCustom2 error = CustomError.Error{msg: "content error 2"}
//   EqError(err1).Matches(err1) // true
//   EqError(err1).Matches(err2) // false
//   EqError(err1).Matches(errCustom1) // true
//   EqError(err1).Matches(errCustom2) // false

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [ x] Includes tests

Reviewer Notes

• [ x] The code flow looks good.
• [ x] Tests added.

No change, just add a Matcher and it was documented and tested

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[ymohl-cl]

@googlebot I signed it!

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[codyoss]

See discussion on linked issue.