String: Display regex error on invalid group reference (#1537)

plugins/String/plugin.py

@@ -218,6 +218,7 @@ class String(callbacks.Plugin):
         s/regexp/replacement/flags, returns the result of applying such a
         regexp to <text>.
         """
+        try:
             if f('') and len(f(' ')) > len(f(''))+1: # Matches the empty string.
                 s = _('You probably don\'t want to match the empty string.')
                 irc.error(s)