Incorrect error message when Paintera doesn't have write permissions over project directory

[cmhulbert]

If you provide Paintera with a project directory that the user doesn't have write access to, Paintera acts as if the project directory is locked. If you try to Ignore Lock it has no effect, and Paintera exits. This happens in the following places:

1. A user tries passing in a project directory to Paintera directory that they don't have write access to
2. A user tries saving the project to a directory they don't have access to
3. A user tries passing in a project directory that does not exist, in a location they do not have write access to.

Ideally, Paintera should provide a useful message to the user that the user doesn't have appropriate permissions for the desired project location.