terraform: GraphWalkerPanicwrap to catch panics, shadow graph uses it

[mitchellh]

Related to #9840

This PR:

• Adds the ability for GraphWalkers to declare they want to catch panics (by implementing GraphWalkerPanicwrapper)

• Modifies the graph walk to turn panics into errors if the above interface is implemented. This makes it so that panics are never able to be completely thrown away. They either crash Terraform or turn into errors that will bubble up.

• Modifies shadow graph execution to always capture panics.

This adds some additional robustness around shadow graph execution so that panics during graph walking do not cause Terraform to crash. If this was in place for #9840 then the panic would've shown up as a normal shadow error versus crashing. This would've been okay since the original (non-shadow) graph finished successfully.

This doesn't cover 100% of the scenarios where a shadow graph can crash, but it covers a very large section and one of two most likely areas it would crash (evaluation). The other area would be graph construction itself. This PR doesn't affect the latter positively or negatively.

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