QtWebEngine: Fix crash with userscript + selection

Fixes #1878

Unfortunately it seems impossible to implement a test for this, as
selection via javascript somehow doesn't trigger this.

doc/userscripts.asciidoc

@@ -42,7 +42,8 @@ In `command` mode:
 - `QUTE_URL`: The current URL.
 - `QUTE_TITLE`: The title of the current page.
 - `QUTE_SELECTED_TEXT`: The text currently selected on the page.
-- `QUTE_SELECTED_HTML` The HTML currently selected on the page.
+- `QUTE_SELECTED_HTML` The HTML currently selected on the page (not supported
+  with QtWebEngine).
 
 In `hints` mode:
 

qutebrowser/browser/browsertab.py

@@ -60,6 +60,11 @@ class WebTabError(Exception):
     """Base class for various errors."""
 
 
+class UnsupportedOperationError(WebTabError):
+
+    """Raised when an operation is not supported with the given backend."""
+
+
 class TabData:
 
     """A simple namespace with a fixed set of attributes.

qutebrowser/browser/commands.py

@@ -1089,7 +1089,10 @@ class CommandDispatcher:
         tab = self._tabbed_browser.currentWidget()
         if tab is not None and tab.caret.has_selection():
             env['QUTE_SELECTED_TEXT'] = tab.caret.selection()
-            env['QUTE_SELECTED_HTML'] = tab.caret.selection(html=True)
+            try:
+                env['QUTE_SELECTED_HTML'] = tab.caret.selection(html=True)
+            except browsertab.UnsupportedOperationError:
+                pass
 
         # FIXME:qtwebengine: If tab is None, run_async will fail!
 

qutebrowser/browser/webengine/webenginetab.py

@@ -171,7 +171,7 @@ class WebEngineCaret(browsertab.AbstractCaret):
 
     def selection(self, html=False):
         if html:
-            raise NotImplementedError
+            raise browsertab.UnsupportedOperationError
         return self._widget.selectedText()
 
     def follow_selected(self, *, tab=False):