Bridge: Rename errorNo() to errorCode(), errorMsg() to errorMessage()

Old method are kept for backward compatible.
Bridge: Rename genSql() to generateSql()

Also genSqlPrepared() is renamed to generateSqlPrepared().

Method executeGenSql() is removed too.
Db: New class DbDiff

This class is split from class/mvc-module.php.
Bridge: Add 'get' prefix to method errorCode(), errorMessage()
Base: Refactor class ReturnValue for easy use and read

Use code, message, data as property instead of store them in info error is
easier to use, the ReturnValue instance can converted to json string and use
loadJson() method to restore, the info array is needless.

Origin getSetInfo() and other code() method with both get and set feature is
a bad design, a method should only do one thing, get OR set, mix their mission
will confuse reader and a little more complicated.