bundle2: add some debugging information to the not-a-bundle error

I found this useful while trying to debug wireproto-related issues.

mercurial/bundle2.py

@@ -678,6 +678,9 b' def getunbundler(ui, fp, magicstring=Non'
         magicstring = changegroup.readexactly(fp, 4)
     magic, version = magicstring[0:2], magicstring[2:4]
     if magic != 'HG':
+        ui.debug(
+            "error: invalid magic: %r (version %r), should be 'HG'\n"
+            % (magic, version))
         raise error.Abort(_('not a Mercurial bundle'))
     unbundlerclass = formatmap.get(version)
     if unbundlerclass is None: