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/test/rpc_tests.cpp

@@ -39,8 +39,7 @@ Value CallRPC(string args)
         Value result = (*method)(params, false);
         return result;
     }
-    catch (Object& objError)
-    {
+    catch (const Object& objError) {
         throw runtime_error(find_value(objError, "message").get_str());
     }
 }