Add custom HardFault handler and catch NaNs

[calebchalmers]

NaNs result from things like dividing by zero and can silently causes a lot of issues. We should try to raise exceptions in these cases, if possible, to make the issue as obvious as possible. The same thing goes for other unhandled exceptions. Currently, the Dev Board will silently crash and this makes debugging difficult. Simply beeping the buzzer in a custom HardFault handler is likely a good way to notify the operator when something is wrong.