bundle2: raise a more helpful error if building a bundle part header fails

I've tripped on this several times now, and am tired of debugging. Now
the header parts are part of the error message when the ''.join()
fails, which makes debugging obvious.

mercurial/bundle2.py

@@ -1050,7 +1050,11 b' class bundlepart(object):'
             header.append(key)
             header.append(value)
         ## finalize header
-        headerchunk = ''.join(header)
+        try:
+            headerchunk = ''.join(header)
+        except TypeError:
+            raise TypeError(r'Found a non-bytes trying to '
+                            r'build bundle part header: %r' % header)
         outdebug(ui, 'header chunk size: %i' % len(headerchunk))
         yield _pack(_fpartheadersize, len(headerchunk))
         yield headerchunk