Restore ValidationError as part of the werkzeug.routing API

This matches the documented import path.

Author: pgjones

src/werkzeug/routing/__init__.py

@@ -112,6 +112,7 @@
 from .converters import PathConverter
 from .converters import UnicodeConverter
 from .converters import UUIDConverter
+from .converters import ValidationError
 from .exceptions import BuildError
 from .exceptions import NoMatch
 from .exceptions import RequestAliasRedirect