Incorrect examples in development/errors.html

[chinpei215]

1. http://book.cakephp.org/3.0/en/development/errors.html#creating-your-own-error-handler
_displayError() and _displayException() declared as @return void. Returning string doesn't make sense.

2. http://book.cakephp.org/3.0/en/development/errors.html#changing-fatal-error-behavior
handleFatalError() is declared as @return bool. Returning string doesn't make sense.

[markstory]

Looks like the docs were not correctly updated from the 2.x days. 😢

[chinpei215]

Half-solved in #4694.