convert: fail properly if we can't read a source hg repository

hgext/convert/hg.py

@@ -156,7 +156,10 b' class mercurial_sink(converter_sink):'
 class mercurial_source(converter_source):
     def __init__(self, ui, path, rev=None):
         converter_source.__init__(self, ui, path, rev)
-        self.repo = hg.repository(self.ui, path)
+        try:
+            self.repo = hg.repository(self.ui, path)
+        except:
+            raise NoRepo("could not open hg repo %s as source" % path)
         self.lastrev = None
         self.lastctx = None