Catch exceptions in Transformer construction

[sundbry]

Summary

When transformerPath is set in metro.config.js, if there is a failure in loading the custom transformer, the exception is silently swallowed and all the user sees is this misleading error message (see below). In this patch, we add a catch to the promise to report the original exception to the user.

Test plan

Set an invalid transformerPath in metro.config.js to reproduce:

module.exports = {
  transformerPath: require.resolve('./fail.js'),
}

Cannot read property 'transformFile' of undefined

TypeError: Cannot read property 'transformFile' of undefined
    at /Users/go/Library/Application Support/Go Agent/pipelines/ngf-app-ios/node_modules/react-native/node_modules/metro/src/Bundler.js:83:34
    at Generator.next (<anonymous>)
    at asyncGeneratorStep (/Users/go/Library/Application Support/Go Agent/pipelines/ngf-app-ios/node_modules/react-native/node_modules/metro/src/Bundler.js:14:24)
    at _next (/Users/go/Library/Application Support/Go Agent/pipelines/ngf-app-ios/node_modules/react-native/node_modules/metro/src/Bundler.js:34:9)

[codecov-io]

Codecov Report

Merging #351 into master will increase coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   85.01%   85.02%   +<.01%     
==========================================
  Files         171      172       +1     
  Lines        5205     5221      +16     
  Branches      793      795       +2     
==========================================
+ Hits         4425     4439      +14     
- Misses        693      695       +2     
  Partials       87       87

Impacted Files	Coverage Δ
packages/metro/src/Bundler.js	92.3% <66.66%> (-7.7%)	⬇️
...ackages/buck-worker-tool/src/CommandFailedError.js	50% <0%> (ø)
packages/buck-worker-tool/src/worker-tool.js	95.18% <0%> (+0.89%)	⬆️

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[rafeca]

Thanks for the contribution!

[facebook-github-bot]

@rafeca is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.