bundle2: also capture reply capability on failure

When unbundling over the wire is aborted, we have a mechanism to convey the
error inside a bundle part. As we add support for more errors, we need to know if
the client will support them. For this purpose, we duck punch the reply
capabilities of the client on the raised extensions.

This is similar to what is done to salvage the server output on error.

mercurial/bundle2.py

@@ -347,8 +347,11 b' def processbundle(repo, unbundler, trans'
         # craziness in a future version.
         exc.duringunbundle2 = True
         salvaged = []
+        replycaps = None
         if op.reply is not None:
             salvaged = op.reply.salvageoutput()
+            replycaps = op.reply.capabilities
+        exc._replycaps = replycaps
         exc._bundle2salvagedoutput = salvaged
         raise
     finally: