Don't throw a error when writing JSON fails

doc/lua_api.txt

@@ -1531,7 +1531,7 @@ minetest.parse_json(string[, nullvalue]) -> something
 ^ On success returns a table, a string, a number, a boolean or nullvalue
 ^ On failure outputs an error message and returns nil
 ^ Example: parse_json("[10, {\"a\":false}]") -> {10, {a = false}}
-minetest.write_json(data[, styled]) -> string
+minetest.write_json(data[, styled]) -> string or nil and error message
 ^ Convert a Lua table into a JSON string
 ^ styled: Outputs in a human-readable format if this is set, defaults to false
 ^ Un-serializable things like functions and userdata are saved as null.

src/script/common/c_content.cpp

@@ -1106,26 +1106,26 @@ void get_json_value(lua_State *L, Json::Value &root, int index)
 			if (keytype == LUA_TNUMBER) {
 				lua_Number key = lua_tonumber(L, -1);
 				if (roottype != Json::nullValue && roottype != Json::arrayValue) {
-					throw LuaError(NULL, "Can't mix array and object values in JSON");
+					throw SerializationError("Can't mix array and object values in JSON");
 				} else if (key < 1) {
-					throw LuaError(NULL, "Can't use zero-based or negative indexes in JSON");
+					throw SerializationError("Can't use zero-based or negative indexes in JSON");
 				} else if (floor(key) != key) {
-					throw LuaError(NULL, "Can't use indexes with a fractional part in JSON");
+					throw SerializationError("Can't use indexes with a fractional part in JSON");
 				}
 				root[(Json::ArrayIndex) key - 1] = value;
 			} else if (keytype == LUA_TSTRING) {
 				if (roottype != Json::nullValue && roottype != Json::objectValue) {
-					throw LuaError(NULL, "Can't mix array and object values in JSON");
+					throw SerializationError("Can't mix array and object values in JSON");
 				}
 				root[lua_tostring(L, -1)] = value;
 			} else {
-				throw LuaError(NULL, "Lua key to convert to JSON is not a string or number");
+				throw SerializationError("Lua key to convert to JSON is not a string or number");
 			}
 		}
 	} else if (type == LUA_TNIL) {
 		root = Json::nullValue;
 	} else {
-		throw LuaError(NULL, "Can only store booleans, numbers, strings, objects, arrays, and null in JSON");
+		throw SerializationError("Can only store booleans, numbers, strings, objects, arrays, and null in JSON");
 	}
 	lua_pop(L, 1); // Pop value
 }

src/script/lua_api/l_util.cpp

@@ -179,7 +179,7 @@ int ModApiUtil::l_parse_json(lua_State *L)
 	return 1;
 }
 
-// write_json(data[, styled]) -> string
+// write_json(data[, styled]) -> string or nil and error message
 int ModApiUtil::l_write_json(lua_State *L)
 {
 	NO_MAP_LOCK_REQUIRED;
@@ -191,7 +191,13 @@ int ModApiUtil::l_write_json(lua_State *L)
 	}
 
 	Json::Value root;
-	get_json_value(L, root, 1);
+	try {
+		get_json_value(L, root, 1);
+	} catch (SerializationError &e) {
+		lua_pushnil(L);
+		lua_pushstring(L, e.what());
+		return 2;
+	}
 
 	std::string out;
 	if (styled) {