gnulib-tool.py: Don't continue creating testdirs when destdir exists. * pygnulib/GLError.py (GLError.__init__, GLError.__repr__): Add a new error number for destination directories that already exist. * pygnulib/main.py (main_with_exception_handling): Print the message. * pygnulib/GLTestDir.py (GLTestDir.__init__, GLMegaTestdir.__init__): Fail if the destination directory exists instead of creating files and failing to patch test driver.