Propagate Bundler transformer initialization errors

[Boulea7]

Summary

Fixes #1808.

Preserve and propagate the original Transformer construction error after reporting transformer_load_failed. Attach an immediate observer to the initialization promise so a caller can await the same rejected promise later without an unhandled rejection.

Changelog: [Fix] Propagate Transformer initialization errors from Bundler

Test plan

• RED: yarn jest packages/metro/src/__tests__/Bundler-test.js --runInBand failed on 236ea72f1: ready() did not reject and transformFile() surfaced the follow-on undefined-transformer error.
• GREEN: yarn jest packages/metro/src/__tests__/Bundler-test.js --runInBand (3 tests passed).
• yarn jest --runInBand (141 suites, 2533 tests passed).
• yarn typecheck
• yarn lint
• yarn build

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