`TaffyTree` methods should have better error handling

[alice-i-cecile]

Yeah, IMO this should either be changed to return Err/None if the node doesn't exist. Or just return the value directly.

Originally posted by @nicoburns in #765 (comment)

[nicoburns]

See also #520 which has the right idea but takes things slightly too far.

[nicoburns]

I think we ought to:

• Have one set of methods (with unprefixed names) that panic if e.g. the NodeId is missing
• Have a second set of methods (prefixed with something like get_ and/or try_) that return Result or Option and actually return Err/None on failure (and never panic).

[alice-i-cecile]

I'm alright with that. Let's use the try_ prefix. I think this is old tech debt from the stretch days!