unbundle: test and fix for clean abort on unknown bundle2 feature

This introduce a test for the feature and fix the exception name that is
different on default.

mercurial/commands.py

@@ -6388,7 +6388,7 b' def unbundle(ui, repo, fname1, *fnames, '
                 try:
                     op = bundle2.processbundle(repo, gen, lambda: tr)
                     tr.close()
-                except error.UnsupportedPartError as exc:
+                except error.BundleUnknownFeatureError as exc:
                     raise util.Abort(_('%s: unknown bundle feature, %s')
                                      % (fname, exc),
                                      hint=_("see https://mercurial.selenic.com/"

tests/test-bundle2-format.t

@@ -1219,5 +1219,9 b' unknown compression while unbundling'
   $ cat ../rev.hg2.bz | hg statbundle2
   abort: unknown parameters: Stream Parameter - Compression='FooBarUnknown'
   [255]
+  $ hg unbundle ../rev.hg2.bz
+  abort: ../rev.hg2.bz: unknown bundle feature, Stream Parameter - Compression='FooBarUnknown'
+  (see https://mercurial.selenic.com/wiki/BundleFeature for more information)
+  [255]
 
   $ cd ..