svn_swig_wrapper: improved handling of missing or outdated bindings. Instead of aborting with a generic message when Subversion bindings are missing, provide a helpful message. Also, the version check is refactored to make it easier to bump our requirements in the future. Finally, error messages are shorten so they fit in 80 columns along with the standard `abort: ' prefix.

import imp, os, sys

sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))