gracefully handle failures to save passwords

Saving passwords to the OS X keychain can fail, for example when
running in an SSH session.

mercurial_keyring.py

@@ -388,7 +388,11 b' class HTTPPasswordHandler(object):'
             # Otherwise we won't be able to find the password so it
             # does not make much sense to preserve it
             _debug(ui, _("Saving password for %s to keyring") % user)
-            password_store.set_http_password(final_url, user, pwd)
+            try:
+                password_store.set_http_password(final_url, user, pwd)
+            except keyring.errors.PasswordSetError, e:
+                ui.traceback()
+                ui.warn(_("warning: failed to save password in keyring\n"))
 
         # Saving password to the memory cache
         self.pwd_cache.store(realm, final_url, user, pwd)