raise an exception when unable to open zone file

git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1216 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/python/isc/auth/master.py

@@ -345,7 +345,7 @@ def openzone(filename, initial_origin = '.'):
     try:
         zf = open(filename, 'r')
     except:
-        return
+        raise MasterFileError("Could not open " + filename)
     origin = initial_origin
     return zf