Disallow register mode for :enter-mode

doc/changelog.asciidoc

@@ -72,6 +72,8 @@ Fixed
   URLs containing the match as substring. This is now fixed.
 - When a filename is derived from a page's title, it's now shortened to the
   maximum filename length permitted by the filesystem.
+- `:enter-mode register` crashed since v1.13.0, it now displays an error
+  instead.
 
 v1.13.1 (2020-07-17)
 --------------------

qutebrowser/keyinput/modeman.py

@@ -395,7 +395,8 @@ class ModeManager(QObject):
             raise cmdutils.CommandError("Mode {} does not exist!".format(mode))
 
         if m in [usertypes.KeyMode.hint, usertypes.KeyMode.command,
-                 usertypes.KeyMode.yesno, usertypes.KeyMode.prompt]:
+                 usertypes.KeyMode.yesno, usertypes.KeyMode.prompt,
+                 usertypes.KeyMode.register]:
             raise cmdutils.CommandError(
                 "Mode {} can't be entered manually!".format(mode))