Try to detect if somethings wrong with the provided project

[ap--]

from @vankhoa21991

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-10-1c484506e8c5> in <module>
----> 1 qp = QuPathProject('./my_qupath_project', mode="a")
      2 qp.images  # <- access images via this
c:\somewhere\github\paquo\paquo\projects.py in __init__(self, path, mode, image_provider)
    197             for f in p_dir.iterdir():
    198                 if not f.match("*.backup"):
--> 199                     raise ValueError("will only create projects in empty directories")
    200             project = Projects.createProject(File(str(p_dir)), BufferedImage)
    201 
ValueError: will only create projects in empty directories

The project was a folder with images already added but no project.qpproj file.
Likely from a previous run, where QuPathProject.save wasn't called.

There should be a better error message in case this happens.