Add warning message when exporting svg from workflow

[cjsha]

When exporting SVGs, I think it could be helpful to have a warning message if a workflow does not compile or the environment doesn't contain all the necessary packages to run the workflow (which creates hatched nodes in the exported SVG).

[glopesdev]

@cjsha this is a good idea. I am wondering whether we should go further and actually fail the call with an error, since for such purposes the resulting export will most likely never be quite useful.

That way CI systems could more easily signal a failure in their pipelines.

[cjsha]

Should it be an option to elevate from warning to error? I can imagine a situation in which you want to export an uncompilable workflow to discuss specific reasons why a workflow might not compile and how Bonsai behaves in those situations