wrong error message on pdf generation failure

[ael-code]

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On the error handler used to deal with html-to-pdf compilation errors, the same error message pdfGeneration is used both in the case we are using wkhtmltopdf and phantom. The problem is that the error message is the following:

PDF generation failed. Make sure wkhtmltopdf is installed and accessible from your path

So it should be used only when the engine in use is wkhtmltopdf.

Moreover the original error raised by the underlying engine is not printed at all. And this make difficult the debugging process.