Ensure the project folder exists on disk before setting as maya project folder

[mbrainerd]

The project folder may not exist by the time this method is called (i.e. if the schema folder is set to "defer_creation" and we are changing from another context). This small fix just ensures that it exists before setting it as Maya's project folder, which fails and errors out the process if not.

[mbrainerd]

Any update on this request?