tpl::loadFile now checks that `$vars` is an array

When using an error control operator, errors are still propagated to custom error handlers. This particular error is simple enough that it's worth avoiding altogether.

    extract() expects parameter 1 to be array, boolean given C:\...path...\kirby\lib\template.php 31
Merge pull request #69 from zzzzBov/patch-2

tpl::loadFile now checks that `$vars` is an array