Implemented error handler

[bantya]

Fixes #11

[feat7]

Added whoops?
Can you provide more information about what you've done?
@bantya

[bantya]

filp/whoops is a popular package to deal with errors/exceptions in php. This package was part of Laravel framework for ~2 versions.

I required this package using Composer and added initial setup steps in index.php. (These steps are given in its README file.) I enabled this package only for __dev__ environment, i.e. the error handler will only work when this project is decided to run in development mode.

Once this setup is done, you can catch the errors/exception.