Add an erroneous file for triggering failure cases

[bizurkur]

This allows for setting up error messages for basic file actions, such as: open, read, write, truncate, tell, seek, stat, eof, and lock.

Example:

$root = vfsStream::setup();
$file = vfsStream::newErronousFile('foo.txt', ['open' => 'error message'])->at($root);

fopen($file->url(), 'w');
// error message

Closes #155

[mikey179]

I think overall it's good and the right way to go.