Fix for check_pydot when Graphviz is not installed

[harahu]

Currently, if pydot is installed, but Graphviz is not, check_pydot will raise a pydot.InvocationException instead of returning False. This PR should fix this issue.

Should probably add a test to confirm the issue, but I wasn't entirely sure how best to go about it, so holding off on it for now.

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[harahu]

Dug into pydota bit and found this: https://github.com/pydot/pydot-ng/blob/8817b2141008c4b74507efb1b7711da7caa77684/test/test_pydot.py#L197-L204

Might want to do a test based on the patching pattern found there to confirm this.