Don't catch every ValueError to get failed interpolations

qutebrowser/app.py

@@ -180,11 +180,11 @@ class Application(QApplication):
             self.config = ConfigManager(confdir, 'qutebrowser.conf', self)
         except (config.ValidationError,
                 config.NoOptionError,
+                config.InterpolationSyntaxError,
                 configparser.InterpolationError,
                 configparser.DuplicateSectionError,
                 configparser.DuplicateOptionError,
-                configparser.ParsingError,
-                ValueError) as e:
+                configparser.ParsingError) as e:
             errstr = "Error while reading config:"
             if hasattr(e, 'section') and hasattr(e, 'option'):
                 errstr += "\n\n{} -> {}:".format(e.section, e.option)

qutebrowser/config/config.py

@@ -71,6 +71,13 @@ class NoOptionError(configparser.NoOptionError):
     pass
 
 
+class InterpolationSyntaxError(ValueError):
+
+    """Exception raised when configparser interpolation raised an error."""
+
+    pass
+
+
 class ConfigManager(QObject):
 
     """Configuration manager for qutebrowser.
@@ -361,7 +368,11 @@ class ConfigManager(QObject):
             changed: If the config was changed.
             style_changed: When style caches need to be invalidated.
         """
-        value = self._interpolation.before_set(self, sectname, optname, value)
+        try:
+            value = self._interpolation.before_set(self, sectname, optname,
+                                                   value)
+        except ValueError as e:
+            raise InterpolationSyntaxError(e)
         try:
             sect = self.sections[sectname]
         except KeyError: