contrib/fast-import/import-zips.py: fix broken error message

The 'sys' module is not imported but all of the bits
we want from it are.  Adjust the script to not fail
when run on old Python versions and fix the inconsistent
use of tabs.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/import-zips.py

@@ -15,8 +15,8 @@
 
 if hexversion < 0x01060000:
         # The limiter is the zipfile module
-        sys.stderr.write("import-zips.py: requires Python 1.6.0 or later.\n")
-        sys.exit(1)
+        stderr.write("import-zips.py: requires Python 1.6.0 or later.\n")
+        exit(1)
 
 if len(argv) < 2:
         print 'usage:', argv[0], '<zipfile>...'