Improve error messages in pptx to pdf conversion

[riyasunil]

The document conversion does not support ppt, pptx documents with transitions to be converted to pdf.
The cloud convert online file conversion service throws a generic error : This file cannot be converted, probably because of read-only fonts.

Using the python-sdk for cloud convert, if you write a job to convert a pptx to pdf (that contains transitions), it throws another non-specific error : Input task has failed

[josiasmontag]

This is a file specific issue and not an issue about the Python SDK. As a customer, you can contact CloudConvert directly here: https://cloudconvert.com/contact