Improves exception classes.

[jcchavezs]

This PR improves the exceptions based on this two references:

• http://blog.nikolaposa.in.rs/2016/08/17/exceptional-behavior-best-practices/
• https://www.alainschlesser.com/structuring-php-exceptions/

Ping @beberlei @felixfbecker

[jcchavezs]

@felixfbecker I just addressed your comment, I agree.