use const char* instead of a std::string in t_translation::error

src/map.cpp

@@ -199,7 +199,7 @@ void gamemap::read(const std::string& data)
 	} catch(t_translation::error& e) {
 		// We re-throw the error but as map error.
 		// Since all codepaths test for this, it's the least work.
-		throw incorrect_map_format_exception(e.message.c_str());
+		throw incorrect_map_format_exception(e.message);
 	}
 
 	// Convert the starting positions to the array

src/terrain_translation.hpp

@@ -24,6 +24,7 @@
 
 namespace t_translation {
 
+
 	typedef Uint32 t_layer;
 	const t_layer WILDCARD = 0x2A000000;
 	const t_layer NO_LAYER = 0xFFFFFFFF;
@@ -94,8 +95,8 @@ namespace t_translation {
 	// Note: atm most thrown result in a crash, but I like
 	// an uncatched exception better than an assert.
 	struct error {
-		error(const std::string& msg) : message(msg) {}
-		std::string message;
+		error(const char* msg) : message(msg) {}
+		const char* const message;
 	};
 
 	// Some types of terrain which must be known, and can't just