Add a check that a non-new project actually exists

[vkbo]

Summary:

The NWStorage class did not actually check that a project folder exists before trying to open a project. Unless it is a new project, the folder should already exists, and if it doesn't, an error returned. This PR fixes this issue.

Related Issue(s):

Closes #1300

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed