Rethrow KeyError as DoesNotExistError in urlmarks.

From @TheCompiler:
To expand on this: I think it's fine to use KeyError on a lower level, i.e.
with the SqlTable object with a dict-like interface. However, on this higher
level, I think it makes sense to re-raise them as more specific exceptions.

qutebrowser/browser/commands.py

@@ -1261,7 +1261,7 @@ class CommandDispatcher:
             url = self._current_url()
             try:
                 quickmark_manager.delete_by_qurl(url)
-            except KeyError as e:
+            except urlmarks.DoesNotExistError as e:
                 raise cmdexc.CommandError(str(e))
         else:
             try:

qutebrowser/browser/urlmarks.py

@@ -53,6 +53,13 @@ class InvalidUrlError(Error):
     pass
 
 
+class DoesNotExistError(Error):
+
+    """Exception emitted when a given URL does not exist."""
+
+    pass
+
+
 class AlreadyExistsError(Error):
 
     """Exception emitted when a given URL does already exist."""
@@ -179,12 +186,14 @@ class QuickmarkManager(UrlMarkManager):
         try:
             self.delete(urlstr, field='url')
         except KeyError:
-            raise KeyError("Quickmark for '{}' not found!".format(urlstr))
+            raise DoesNotExistError("Quickmark for '{}' not found!"
+                                    .format(urlstr))
 
     def get(self, name):
         """Get the URL of the quickmark named name as a QUrl."""
         if name not in self:
-            raise KeyError("Quickmark '{}' does not exist!".format(name))
+            raise DoesNotExistError("Quickmark '{}' does not exist!"
+                                    .format(name))
         urlstr = self[name]
         try:
             url = urlutils.fuzzy_url(urlstr, do_search=False)