Check a disc read/write error and recover from it.
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 Syntax

            bNewHandler := {|e| GT_ErrDisc(e, bOldHandler)}

 Arguments:

      e - Error object
                bOldHandler - Previous Error Handler

 Returns:

                An array of three elements
                Elem[1] := Error Message
                Elem[2] := User Action
                Elem[3] := Error number - useful if you want to
                           divert to another routine e.g. for formatting

 Description:

      Error handler for use with disc read/writes
                to enable recovery from disc problems.
                MUST Be Used within BEGIN SEQUENCE/END SEQUENCE construct

 Examples:

                FUNCTION DiscErr()
                        Local oOldHandler
                        Local nTries := 0
                        Local nAns
                        local aArrErr

                   // Set up the New handler and pass old handler as a parameter
                   bNewHandler := {|e| ErrDisc(e, bOldHandler)}
                   bOldHandler := ErrorBlock(bNewHandler)

                        WHILE (.t.)
                                BEGIN SEQUENCE

                                        // Floppy disc access here
                                        fopen('a:\missing.zxy')

                                        //For low Level functions that don't normally use the
                                        // error system
                                         GT_LOWERR()

                                        // This is the recovery section of the Begin/End Sequence
                                        // construct.  A BREAK statement got us to this point.
                                RECOVER USING aErrArr

                                        // Allow the user three attempts
                                        if nTries++ < 3
                                                ...
                                                ...
                                                ...
                                                // Your recovery code
                                                ...
                                                ...
                                                ...
                                                Do Case
                                                Case nAns == 2
                                                        EXIT
                                                Case nAns == 3
                                                        QUIT
                                                EndCase
                                        ENDIF
                                END SEQUENCE
                        ENDDO
                RETURN NIL

 Header File: GT_Lower.ch

 Source: GT_DERR.PRG

 Author:  Niall Scott