Add a more helpfull error message for when the pdf save location is locked, e.g. by Acrobat Reader

[phseiff]

See also #50 (comment).

[phseiff]

This is implemented in v1.18.0 now; you can get it e.g. via pip3 install --upgrade gh-md-to-html>=1.18.0.

The error text says:

Saving the pdf faile failed because the saving location was unwriteable.
This can be caused by a read-only file system or many other reasons, but the most probable reason is that you had an
older version of the pdf opened in a pdf viewer whilst gh-md-to-html was trying to write the new version to its
location, and that the pdf viewer you used locks pdf files whilst it has them opened. This is standard behavior with
Adobe Reader, for example.
For pdf viewers that can handle live-upading the pdf, see here, for example:
https://superuser.com/questions/599442/pdf-viewer-that-handles-live-updating-of-pdf-doesnt-lock-the-file