open: print better error message when trying to open a session on non-existing workflows

[tiborsimko]

Current behaviour

When a user tries to open a Jupyter notebook session on a non-existing workflow, the error message is:

$ reana-client open -w nonexistent
==> ERROR: Interactive session could not be opened:
The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.

This can be misleading as if there was something wrong with the cluster rather than the client arguments.

Expected behaviour

It will be more user friendly to say that the workflow does not exist, and therefore we cannot open a notebook session on it. Similarly as we do for other commands such as status:

$ reana-client status -w nonexistent
==> ERROR: Cannot retrieve the status of a workflow nonexistent:
REANA_WORKON is set to nonexistent, but that workflow does not exist. Please set your REANA_WORKON environment variable appropriately.