fix poor rethrows

cppcheck (via Codacy) points out that it is better to rethrow the current event than to throw a new copy of it.

src/editor/map/map_context.cpp

@@ -321,7 +321,7 @@ void map_context::load_scenario(const config& game_config)
 		read(scenario, *(preprocess_file(filename_)));
 	} catch(config::error& e) {
 		LOG_ED << "Caught a config error while parsing file: '" << filename_ << "'\n" << e.message << std::endl;
-		throw e;
+		throw;
 	}
 
 	scenario_id_ = scenario["id"].str();

src/formula/formula.cpp

@@ -963,7 +963,7 @@ class string_expression : public formula_expression
 				sub.calculation.reset(new formula(formula_str));
 			} catch(formula_error& e) {
 				e.filename += " - string substitution";
-				throw e;
+				throw;
 			}
 
 			subs_.push_back(sub);