[expo-updates] Expose emergency launch reason on constants

[wschurman]

Why

Previously, the only way to find out why expo-updates may have failed to initialize was to look at the Expo-Fatal-Error header of the manifest request.

This PR adds a second method, a constant in the JS module.

Closes ENG-11739.

How

Expose the error message to a JS field.

Test Plan

Build. We don't currently have tests for these cases because they're rare and hard to reproduce. May look into adding a e2e test where we force an error, but that could prove to be an uphill battle.

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[linear]

ENG-11739 Add Updates.initializationError constant

[expo-bot]

The Pull Request introduced fingerprint changes against the base commit: bb80091

Fingerprint diff

[
  {
    "type": "dir",
    "filePath": "../../packages/expo-updates/android",
    "reasons": [
      "expoAutolinkingAndroid"
    ],
    "hash": "8c8ab657e64a5547ed2a4ec9647ce8b1db38313d"
  }
]

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