Add check to ensure that cert file is not a directory

[mcoles]

Right now, if you pass in a directory rather than a file as your certificate, you get a cryptic "21" error back. We make sure the path is legit, but don't check if if it's a real file, or a directory. That's confusing but should be easily remedied.

Source: https://community.tableau.com/s/question/0D58b0000AG0Ge9CQF/getting-an-error-when-trying-to-generate-a-trusted-ticket