fall back correctly on creating namespace

src/core/main.py

@@ -668,7 +668,9 @@ def main():
 
         for namespace in configuration.evaluateXpath("yep:namespace"):
             name, uri = namespace.prop("name"), namespace.prop("uri")
-            if not namespaces.getUri(name):
+            try:
+                namespaces.getUri(name):
+            except KeyError:
                 ns = namespaces.Namespace(name, uri)
             application.xpathContext.xpathRegisterNs(name, uri)