[TASK] Mark Exception classes as final (#731)

Fixes #709

Author: ghostwriter

src/Parsing/OutputException.php

@@ -7,7 +7,7 @@
 /**
  * Thrown if the CSS parser attempts to print something invalid.
  */
-class OutputException extends SourceException
+final class OutputException extends SourceException
 {
     /**
      * @param string $sMessage

src/Parsing/UnexpectedEOFException.php

@@ -9,4 +9,4 @@
  *
  * Extends `UnexpectedTokenException` in order to preserve backwards compatibility.
  */
-class UnexpectedEOFException extends UnexpectedTokenException {}
+final class UnexpectedEOFException extends UnexpectedTokenException {}