sftp.py causes dput to fail if bzrlib not installed

Binary package hint: dput

the new sftp.py upload method includes the following code:

try:
    import bzrlib.transport
except Exception, e:
    print "E: bzrlib must be installed to use sftp transport."
    sys.exit(1)

This is a problem because dput uses a glob to import *all* the transports, so if bzrlib is not installed this import will fail on every invocation of dput, and bzr is only a recommends of the dput package.

Please fix this to not exit on import failure.