Replace rethrows with typed throws in Graph

[stmontgomery]

This replaces usage of rethrows with typed throws in the internal-only Graph support type.

Motivation:

It's now recommended to use typed throws in generic code that never produces its own errors, but only passes through errors that come from callers. And adopting typed throws in more places will reduce barriers to eventually supporting Embedded Swift.

Modifications:

• Replace all usage of rethrows with the analogous typed throws declaration in Graph.

Result:

No behavioral/runtime change.

Checklist:

• [x] Code and documentation should follow the style of the Style Guide.
• [x] If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[stmontgomery]

@swift-ci please test

[grynspan]

Let's double-check that this is back-deployable to older Apple OSes. Otherwise, ✅

[stmontgomery]

I've gotten confirmation offline that the typed throws is back-deployable.

[stmontgomery]

@swift-ci please test