Add warnings about missing files of model during visualisation

[lej0hn]

When visualising a model, it is possible to produce an incorrect visualisation while not all model files are present without any relevant warning or error.

Describe the solution you'd like
Add warning in the output logs of the visualisation in case of missing files. Maybe the files could be validated before the visualisation process and some corresponding warnings could be produced.

[lej0hn]

Upon reading a neuroml file there is a boolean includes_include that when True, the included files are validated and if there is a missing file a critical log occurs and the process exits. Would it be a good solution to let the validation happen always, but then, it can be decided with another boolean to whether exit the process or just log a warning for the missing file upon finding one?