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Gets the last error associated with the stream.
const boost::system::error_code & error() const;
Return Value
            An error_code corresponding
            to the last error from the stream.
          
Remarks
            Returns a reference to an error_code
            object representing the last failure reported by an istreambuf
            function. The set of possible error_code
            values and categories depends on the protocol of the URL used to open
            the stream buffer.