Make get_biome_list() error message more helpful

src/script/lua_api/l_mapgen.cpp

@@ -416,7 +416,9 @@ size_t get_biome_list(lua_State *L, int index,
 	if (is_single) {
 		Biome *biome = get_or_load_biome(L, index, biomemgr);
 		if (!biome) {
-			errorstream << "get_biome_list: failed to get biome" << std::endl;
+			errorstream << "get_biome_list: failed to get biome '"
+				<< (lua_isstring(L, index) ? lua_tostring(L, index) : "")
+				<< "'." << std::endl;
 			return 1;
 		}
 
@@ -433,8 +435,9 @@ size_t get_biome_list(lua_State *L, int index,
 		Biome *biome = get_or_load_biome(L, -1, biomemgr);
 		if (!biome) {
 			fail_count++;
-			errorstream << "get_biome_list: failed to load biome (index "
-				<< count << ")" << std::endl;
+			errorstream << "get_biome_list: failed to get biome '"
+				<< (lua_isstring(L, -1) ? lua_tostring(L, -1) : "")
+				<< "'" << std::endl;
 			continue;
 		}