[core] Make error widget scrollable

Restrict error module to 15 characters and make it scrollable.

modules/__pulseaudio.py

@@ -113,7 +113,7 @@ class Module(core.module.Module):
         logging.error('no pulseaudio device found')
         return 'n/a'
 
-    def volume(self):
+    def volume(self, widget):
         if self._failed == True:
             return 'n/a'
         if int(self._mono) > 0:

modules/datetime.py

@@ -33,7 +33,7 @@ class Module(core.module.Module):
     def default_format(self):
         return '%x %X'
 
-    def full_text(self):
+    def full_text(self, widget):
         enc = locale.getpreferredencoding()
         retval = datetime.datetime.now().strftime(self._fmt)
         if hasattr(retval, 'decode'):