Show a DOS error message
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 Syntax

        GT_ErNam( <nError> ) --> <cErrorMess>

 Arguments:

      <nError>      A valid DOS error number. Could be the value
                    returned by FERROR()

 Returns:

      <cErrorMess>  The description of the error message.
                    Default "".

 Description:

      Function used to return a text description of a DOS error number up
      to dos error number 88.

 Examples:

      local nErr
      Fopen("exist.not")
      if ( nErr := ferror() ) > 0
          ? GT_ErNam(nErr) --> "File not found"
      endif

 Source: GT_ERNAM.PRG

 Author:  Phillip Hamlyn