feat: safeguard against invalid JSON

It appears that OpenWeatherMap sometimes sent out bad JSON.
Learn from the lesson and do better in this PR.

Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>

weather-widget/weather.lua

@@ -322,6 +322,17 @@ local function worker(user_args)
             return
         end
 
+        if string.match(stdout, '<') ~= nil then
+            if not warning_shown then
+                warning_shown = true
+                widget:is_ok(false)
+                tooltip:add_to_object(widget)
+  
+                widget:connect_signal('mouse::enter', function() tooltip.text = stdout end)
+            end
+            return
+        end
+
         warning_shown = false
         tooltip:remove_from_object(widget)
         widget:is_ok(true)