resolve TODO list

[kamilsk]

• panic("invalid URL " + attr) // TODO set error instead of panic
• // TODO make thread safe
• // TODO it can return error
• // TODO return []Page instead []*Page
• panic(err) // TODO set error instead of panic
• panic("can't find link with URL " + href) // TODO set error instead of panic
• panic("can't find page with URL " + href) // TODO set error instead of panic
• panic("unexpected URL " + href) // TODO set error instead of panic

[kamilsk]

done