Fix warn mode throwing validation errors

[sellimenes]

Fixes #113.

Summary

• Wrap warn-mode validation in a catch path so validation-time errors are emitted as warnings instead of escaping.
• Keep the existing warning behavior for normal validation failures.
• Add regression coverage using a $Shape validation path that previously threw.

Tests

• yarn --cwd packages/flow-runtime test
• yarn --cwd packages/flow-runtime eslint ./src/TypeContext.js ./src/typed.test.js
• git diff --check

Note: full yarn --cwd packages/flow-runtime lint is not clean in this local checkout because of existing fixture lint errors, so I also ran targeted ESLint on the changed files.