TransferManager: fix error-handling

An earlier patch (5ed882c6cd56bd9cdc50a9017a2cb74047e00607) switched from
using CellAdapter#sendMessage to the recently introduced call-back mechanism.
This separates the normal and error response handling into separate call-back
methods.  Unfortunately the patch didn't also refactor the error handling
in TransferManagerHandler so the existing error handling remained in the
successful code-path, where it was never called.

This patch moves the error handling to the error call-back method.  It
tidies up the error messages and reduces the logging somewhat.  The retry
if the delete of a file fails is refactored slightly.

Target: master
Patch: http://rb.dcache.org/r/6889/
Acked-by: Gerd Behrmann