ENH Do not show icon if binary is not available

[emteknetnz]

Issue https://github.com/silverstripeltd/product-issues/issues/613

Code is copied from https://github.com/silverstripe/cwp-pdfexport/blob/1.4/src/Extensions/PdfExportControllerExtension.php#L98

I didn't use a static helper function due to the use of $this->owner->config()->get('wkhtmltopdf_binary');

I think code duplication is fine in this context

[maxime-rainville]

Seems to be mostly doing what it's advertised to do.

One of the ACs stated "and any endpoint for generating PDF returns a suitable error."

If I directly try to navigate to http://sink4x.max.silverstripe.com/home/downloadpdf, I get a 500, because it will still try to generate the PDF. My preference is would be that we explicitly check for the presence of the binary and return a 400 or 404 at that stage.

[emteknetnz]

Have updated, now returns a 400 if the binary is missing when you visit the endpoint