[Existing Sniff] Disallow Error Silencing

[grappler]

Rule type:

Error - (No) Error Control Operator @

Rule:

No PHP or JS errors.

PHP errors should not be silenced

Ref: https://make.wordpress.org/themes/handbook/review/required/#code

To do:

• Document the check in the Theme Review handbook to the Requirements page.
• Add existing <rule ref="Generic.PHP.NoSilencedErrors" /> sniff to the ruleset.

[ernilambar]

@grappler This is currently WARNING. Is not this supposed to be ERROR?

[grappler]

@ernilambar Yes, it should be a error.

[jrfnl]

PR #99 should solve that ;-)