resolve TODO list

[kamilsk]

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

[kamilsk]

done