add display_error_messages option to engine settings

A new option is added to engines to hide error messages from users. It
is called `display_error_messages` and by default it is set to `True`.
If it is set to `False` error messages do not show up on the UI.

Keep in mind that engines are still suspended if needed regardless of
this setting.

Closes #1828

docs/admin/engines.rst

@@ -26,6 +26,8 @@ Safe search   **SS**
 Weigth        **W**
 ------------- ----------- ---------------------------------
 Disabled      **D**
+------------- ----------- ---------------------------------
+Show errors   **DE**
 ============= =========== =================================
 
 Configuration defaults (at built time):
@@ -51,6 +53,7 @@ Configuration defaults (at built time):
         - O
 	- W
 	- D
+	- DE
 
       {% for name, mod in engines.items() %}
 
@@ -67,5 +70,6 @@ Configuration defaults (at built time):
         - {{(mod.offline and "y") or ""}}
         - {{mod.weight or 1 }}
         - {{(mod.disabled and "y") or ""}}
+        - {{(mod.display_error_messages and "y") or ""}}
 
      {% endfor %}

docs/admin/settings.rst

@@ -175,6 +175,9 @@ Engine settings
 ``weigth`` : default ``1``
   Weighting of the results of this engine.
 
+``display_error_messages`` : default ``True``
+  When an engine returns an error, the message is displayed on the user interface.
+
 .. note::
 
    A few more options are possible, but they are pretty specific to some