Do not remove invalid runtime dir, throw exception instead

[bcremer]

I don't like how an "invalid" runtime directory is unliked at the moment (https://github.com/dnoegel/php-xdg-base-dir/blob/master/src/Xdg.php#L105).

Instead I would like to throw an exception like throw new RuntimeException('Unsecure Runtime dir').
See also this example: https://github.com/o11c/rust-xdg/blob/master/src/xdg.rs#L52

public function getRuntimeDir($strict=true) could become public function getRuntimeDir($create = true, $forceSecure = true).

Feedback welcome.

[nubs]

I tend to agree. I wouldn't want something deleting things on my filesystem when I expect it to be mostly readonly.