app: Avoid needless exception to pop

It's better to reply 400 Bad Request on malformed requests and do not pollute
log with exceptions.

kedifa/app.py

@@ -367,10 +367,12 @@ class Kedifa(object):
       if not index:
         index = None
     else:
-      raise ValueError
+      start_response('400 Bad Request', headers_text_plain)
+      return ('Wrong path',)
 
     if not reference:
-      raise ValueError
+      start_response('400 Bad Request', headers_text_plain)
+      return ('Wrong path',)
 
     if environ['REQUEST_METHOD'] == 'PUT':
       # key auth

kedifa/test.py

@@ -332,11 +332,11 @@ class KedifaIntegrationTest(KedifaMixinCaucase, unittest.TestCase):
     # KeDiFa does not support nothing on / so for now it just raises
     # possibly in the future it will become self-describing interface
     self.assertEqual(
-      httplib.INTERNAL_SERVER_ERROR,
+      httplib.BAD_REQUEST,
       result.status_code
     )
     self.assertEqual(
-      'A server error occurred.  Please contact the administrator.',
+      'Wrong path',
       result.text
     )