Exception when processing encrypted PDF files

[GrahamDo]

See attached screenshot.

[GrahamDo]

The following Wiki article has been created. Please update it with information pertinent to end users as more workarounds are discovered or fixes released: https://github.com/GrahamDo/SplitPdf/wiki/Error:-The-PDF-document-is-protected-by-an-encryption-not-supported-by-PDF-Sharp

[GrahamDo]

There is no version of PDF Sharp which will support the encryption algorithm used in a certain file. A human-readable message is at least preferable to a crash.

Made a commit to handle that exception and advise the user to PDF print the document first.