bundle2: add generic debug output regarding generated interruption

If we are about to hide the detailed debug output, we need some generic debug
message to replace it in a concise way.

mercurial/bundle2.py

@@ -804,6 +804,8 b' class bundlepart(object):'
                 yield chunk
         except BaseException, exc:
             # backup exception data for later
+            ui.debug('bundle2-input-stream-interrupt: encoding exception %s'
+                     % exc)
             exc_info = sys.exc_info()
             msg = 'unexpected error: %s' % exc
             interpart = bundlepart('error:abort', [('message', msg)],