Deprecates collaborative flag, with improved error message that makes…

… clear the process is exiting and why (jupyterlab#14774)

Updates error message from fix to [jupyterlab#14774](jupyterlab#14774)

jupyterlab/labapp.py

@@ -897,13 +897,15 @@ def initialize(self, argv=None):
                 import jupyter_collaboration  # noqa
             except ImportError:
                 self.log.critical(
-                    """
+                    """\
+Juptyer Lab cannot start, because `jupyter_collaboration` was configured but cannot be `import`ed.
+
 To enable real-time collaboration, you must install the extension `jupyter_collaboration`.
 You can install it using pip for example:
 
   python -m pip install jupyter_collaboration
 
-This flag is now deprecated and will be removed in JupyterLab v5.
+To run without collaboration, disable collaboration (for example, remove the `--collaborative` flag from the commandline; this flag is now deprecated and will be removed in JupyterLab v5).  To see more ways to adjust the collaborative behavior, see https://jupyterlab-realtime-collaboration.readthedocs.io/en/latest/configuration.html and https://discourse.jupyter.org/t/how-to-disable-jupyter-collaboration-extension/21329/9 .
 """
                 )
                 sys.exit(1)