bundle2: make error testing more modular

We have more than Abort to test.

tests/test-bundle2.t

@@ -904,7 +904,11 b' Setting up'
   > 
   > def _pushbundle2failpart(orig, pushop, bundler):
   >     extradata = orig(pushop, bundler)
+  >     reason = pushop.ui.config('failpush', 'reason', None)
+  >     part = None
+  >     if reason == 'abort':
   >     part = bundle2.bundlepart('test:abort')
+  >     if part is not None:
   >     bundler.addpart(part)
   >     return extradata
   > 
@@ -938,6 +942,11 b' Setting up'
 
 Doing the actual push: Abort error
 
+  $ cat << EOF >> $HGRCPATH
+  > [failpush]
+  > reason = abort
+  > EOF
+
   $ hg -R main push other -r e7ec4e813ba6
   pushing to other
   searching for changes