* Fail when attempting to modify unmodifiable target group parameters
As you can't modify Port, Protocol or VPC id for a target group, fail
when this happens rather than pretending to do it.
One could argue that the target group could be recreated rather than
failing, but this has massive knock on implications to other resources
that depend on the TG (all ASGs would need to be updated, the ELB
listener would need to be updated, etc)
* Use `.get()` instead of direct dictionary access
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