make all catch() arguments const

- I saw this on http://en.cppreference.com/w/cpp/language/try_catch and
  thought it would be a good idea
- also unify used format to better be able to search for exception
  uses in our codebase

src/rpcprotocol.h

@@ -122,8 +122,7 @@ public:
             tcp::resolver::query query(server.c_str(), port.c_str());
             endpoint_iterator = resolver.resolve(query);
 #if BOOST_VERSION >= 104300
-        } catch(boost::system::system_error &e)
-        {
+        } catch (const boost::system::system_error&) {
             // If we at first don't succeed, try blanket lookup (IPv4+IPv6 independent of configured interfaces)
             tcp::resolver::query query(server.c_str(), port.c_str(), resolver_query_base::flags());
             endpoint_iterator = resolver.resolve(query);