regionref highlighter just ignore invalid references

fixes #68

src/function_group.hh

@@ -8,6 +8,11 @@
 namespace Kakoune
 {
 
+struct group_not_found : public runtime_error
+{
+    using runtime_error::runtime_error;
+};
+
 template<typename... Args>
 class FunctionGroup
 {
@@ -39,10 +44,10 @@ public:
         String id(path.begin(), sep_it);
         auto it = m_functions.find(id);
         if (it == m_functions.end())
-            throw runtime_error("no such id: " + id);
+            throw group_not_found("no such id: " + id);
         FunctionGroup* group = it->second.template target<FunctionGroup>();
         if (not group)
-            throw runtime_error("not a group: " + id);
+            throw group_not_found("not a group: " + id);
         if (sep_it != path.end())
             return group->get_group(String(sep_it+1, path.end()), path_separator);
         else

src/highlighters.cc

@@ -806,9 +806,13 @@ HighlighterAndId region_ref_factory(HighlighterParameters params)
 
         auto func = [name](const Context& context, HighlightFlags flags, DisplayBuffer& display_buffer,
                           BufferCoord begin, BufferCoord end)
+        {
+            try
             {
                 HighlighterGroup& ref = DefinedHighlighters::instance().get_group(name, '/');
                 apply_highlighter(context, flags, display_buffer, begin, end, ref);
+            }
+            catch (group_not_found&) {}
         };
 
         return HighlighterAndId("regionref(" + params[0] + "," + params[1] + "," + name + ")",