Handle missing Graphviz in .workflow.make_click_command()

[khaeru]

This line:

message-ix-models/message_ix_models/workflow.py

Lines 366 to 370 in 5afc933

	if not go:
	path = context.get_local_path(f"{slug}-workflow.svg")
	wf.visualize(str(path))
	log.info(f"Workflow diagram written to {path}")
	return

…will error if the user has not installed Graphviz. (Note that the Python graphviz package is installed as a dependency, but the user must separately install Graphviz.)

We can:

• Catch graphviz' ExecutableNotFoundError and continue; perhaps log an error with a pointer to the docs. This could also be done upstream, in genno.
• Update the install docs and/or optional dependencies to include a recommendation and link to install Graphviz.

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