revlog: add an exception hint when processing LFS flags without the extension

It would be even better if this was either detected sooner, or the transaction
completed (especially since the read/write processors aren't needed for the
exchange).  But this makes it easier for the user to resolve until that can be
figured out.

mercurial/revlogutils/flagutil.py

@@ -176,8 +176,12 b' def _processflagsfunc(revlog, text, flag'
             vhash = True
 
             if flag not in revlog._flagprocessors:
+                hint = None
+                if flag == REVIDX_EXTSTORED:
+                    hint = _(b"the lfs extension must be enabled")
+
                 message = _(b"missing processor for flag '%#x'") % flag
-                raise revlog._flagserrorclass(message)
+                raise revlog._flagserrorclass(message, hint=hint)
 
             processor = revlog._flagprocessors[flag]
             if processor is not None:

tests/test-lfs-serve.t

@@ -314,6 +314,7 b' TODO: fail more gracefully.'
   transaction abort!
   rollback completed
   abort: missing processor for flag '0x2000'
+  (the lfs extension must be enabled)
   [50]
   $ hg debugrequires -R $TESTTMP/client4_pull/ | grep 'lfs'
   [1]