fix: avoid throwing in warn mode when validation internals fail

[ncthuc2004]

Summary

• prevent warn mode from throwing when validation internals raise invariant errors
• catch thrown errors in TypeContext.warn(...) and emit them through emitWarningMessage(...)
• add a regression test covering $Shape<T> misuse ($Shape<string>) in warn mode

Why

Issue #113 reports that warn: true still throws in some paths (for example $ShapeType invariants), which breaks the expected non-throw warning behavior.

Validation

• ran targeted test:
  • npx mocha --require @babel/polyfill --require ./test-polyfill.js src/warn.test.js
• result: passing (169 passing)

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #113: Errors are sometimes thrown with "warn" option

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