convert/svn: abort operation when python bindings are not available

Subversion python bindings check was not present in svn_sink source
class which made it fail while using svn as destination repository.
Added a more maintainble svn bindings check for svn_source and svn_sink
classes.

hgext/convert/subversion.py

@@ -36,7 +36,7 b' try:'
             category=DeprecationWarning)
 
 except ImportError:
-    pass
+    svn = None
 
 class SvnPathNotFound(Exception):
     pass
@@ -209,11 +209,8 b' class svn_source(converter_source):'
                 issvnurl(ui, url)):
             raise NoRepo(_("%s does not look like a Subversion repository")
                          % url)
-
-        try:
-            SubversionException
-        except NameError:
-            raise MissingTool(_('Subversion python bindings could not be loaded'))
+        if svn is None:
+            raise MissingTool(_('Could not load Subversion python bindings'))
 
         try:
             version = svn.core.SVN_VER_MAJOR, svn.core.SVN_VER_MINOR
@@ -960,6 +957,9 b' class svn_sink(converter_sink, commandli'
         return self.join('hg-authormap')
 
     def __init__(self, ui, path):
+
+        if svn is None:
+            raise MissingTool(_('Could not load Subversion python bindings'))
         converter_sink.__init__(self, ui, path)
         commandline.__init__(self, ui, 'svn')
         self.delete = []