Show errors on invalid --mode argument for :hint

doc/changelog.asciidoc

@@ -93,6 +93,7 @@ Fixed
   be shown for more files such as `qute://help/userscripts.html`.
 - Crash when doing initial run on Wayland without XWayland
 - Crash when trying to load an empty session file.
+- `:hint` with an invalid `--mode=` value now shows a proper error.
 
 Removed
 ~~~~~~~

qutebrowser/browser/hints.py

@@ -32,7 +32,7 @@ import attr
 from PyQt5.QtCore import pyqtSlot, QObject, Qt, QUrl
 from PyQt5.QtWidgets import QLabel
 
-from qutebrowser.config import config
+from qutebrowser.config import config, configexc
 from qutebrowser.keyinput import modeman, modeparsers
 from qutebrowser.browser import webelem
 from qutebrowser.commands import userscripts, cmdexc, cmdutils, runners
@@ -726,6 +726,12 @@ class HintManager(QObject):
 
         if mode is None:
             mode = config.val.hints.mode
+        else:
+            opt = config.instance.get_opt('hints.mode')
+            try:
+                opt.typ.to_py(mode)
+            except configexc.ValidationError as e:
+                raise cmdexc.CommandError("Invalid mode: {}".format(e))
 
         self._check_args(target, *args)
         self._context = HintContext()

tests/end2end/features/hints.feature

@@ -15,6 +15,10 @@ Feature: Using hints
         And I hint with args "links normal" and follow xyz
         Then the error "No hint xyz!" should be shown
 
+    Scenario: Using :hint with invalid mode.
+        When I run :hint --mode=foobar
+        Then the error "Invalid mode: Invalid value 'foobar' - valid values: number, letter, word" should be shown
+
     ### Opening in current or new tab
 
     Scenario: Following a hint and force to open in current tab.