bookmark: informs of failure to upgrade a bookmark

When we explicitly requested to update a bookmark but the bookmark location was
missing locally, we used to silently ignore the case. We now issue a message
about it to point that something wrong is going on.

By chance, we fixed all the cases where is case happened (for explicit pulling
only, issue4700 is still open). But I think it is still valuable to have a
warning in place in case such issue is reintroduced.

This patch have been tested against issue4689 test (but without issue4689 fix).
It give the better but expected failure seen below:

> --- /home/pyd/src/mercurial-dev/tests/test-bookmarks-pushpull.t
> +++ /home/pyd/src/mercurial-dev/tests/test-bookmarks-pushpull.t.err
> @@ -337,12 +337,12 @@
>    adding manifests
>    adding file changes
>    added 1 changesets with 1 changes to 1 files
> -  updating bookmark Y
> +  remote bookmark Y point to locally missing 0d60821d2197
>    (run 'hg update' to get a working copy)
>    $ hg book
>     * @                         1:0d2164f0ce0d
>       X                         1:0d2164f0ce0d
> -     Y                         5:35d1ef0a8d1b
> +     Y                         4:b0a5eff05604
>       Z                         1:0d2164f0ce0d
>
>  Update a bookmark right after the initial lookup -r (issue4700)
> @@ -387,12 +387,11 @@
>    adding manifests
>    adding file changes
>    added 1 changesets with 1 changes to 1 files
> -  updating bookmark Y
>    (run 'hg update' to get a working copy)
>    $ hg book
>     * @                         1:0d2164f0ce0d
>       X                         1:0d2164f0ce0d
> -     Y                         6:0d60821d2197
> +     Y                         4:b0a5eff05604
>       Z                         1:0d2164f0ce0d
>    $ hg -R $TESTTMP/pull-race book
>       @                         1:0d2164f0ce0d

mercurial/bookmarks.py

@@ -401,6 +401,11 b' def updatefromremote(ui, repo, remotemar'
         if scid in repo: # add remote bookmarks for changes we already have
             changed.append((b, bin(scid), status,
                             _("adding remote bookmark %s\n") % (b)))
+        elif b in explicit:
+            explicit.remove(b)
+            ui.warn(_("remote bookmark %s points to locally missing %s\n")
+                    % (b, scid[:12]))
+
     for b, scid, dcid in advsrc:
         changed.append((b, bin(scid), status,
                         _("updating bookmark %s\n") % (b)))
@@ -427,6 +432,11 b' def updatefromremote(ui, repo, remotemar'
             explicit.discard(b)
             changed.append((b, bin(scid), status,
                             _("importing bookmark %s\n") % (b)))
+    for b, scid, dcid in differ:
+        if b in explicit:
+            explicit.remove(b)
+            ui.warn(_("remote bookmark %s points to locally missing %s\n")
+                    % (b, scid[:12]))
 
     if changed:
         tr = trfunc()