Use `error.message` to set the `description` property of a nested error payload

Summary:
Depends on #124.

 ---

**Summary**
Metro reports errors using a JSON payload that has an `errors` array. Each item in this array has a `description` field. For transform errors, this field was set using the value in `error.description` -- however, JS Error objects only have a `message` field. (Grepping the Metro code, no errors (except in one test) ever get a `description` field.) This commit uses `error.message` instead of `error.description` when creating JSON payloads.

```
$ git grep description -- 'packages/**/*.js'
packages/metro/src/JSTransformer/__tests__/Transformer-test.js:        babelError.description = message;
packages/metro/src/lib/formatBundlingError.js:    description: string,
packages/metro/src/lib/formatBundlingError.js:): {type: string, message: string, errors: Array<{description: string}>} {
packages/metro/src/lib/formatBundlingError.js:      errors: [{description: message}],
packages/metro/src/lib/formatBundlingError.js:        description: error.message,
packages/metro/src/node-haste/__tests__/Module-test.js:  description: "A require('foo') story",
```

**Test Plan**
Added a unit test to check that the description field is set for transform errors (with the delta bundler).

Also in a test RN app, inspected the error payload that is received by RN when there's a syntax error with HMR turned on and verified that `data.body.errors[0].description` was set.
Closes https://github.com/facebook/metro/pull/125

Differential Revision: D6730671

Pulled By: rafeca

fbshipit-source-id: 58311462db9223d65580d77748203d8ea0ea1ac7

packages/metro/src/HmrServer/__tests__/HmrServer-test.js

@@ -158,6 +158,7 @@ describe('HmrServer', () => {
         message: 'test syntax error',
         errors: [
           {
+            description: 'test syntax error',
             filename: 'EntryPoint.js',
             lineNumber: 123,
           },

packages/metro/src/JSTransformer/__tests__/Transformer-test.js

@@ -87,7 +87,6 @@ describe('Transformer', function() {
         const babelError = new SyntaxError(message);
 
         babelError.type = 'SyntaxError';
-        babelError.description = message;
         babelError.loc = {line: 2, column: 15};
         babelError.codeFrame = snippet;
 

packages/metro/src/Server/__tests__/Server-test.js

@@ -320,6 +320,7 @@ describe('processRequest', () => {
             message: 'test syntax error',
           });
           expect(body.errors).toContainEqual({
+            description: 'test syntax error',
             filename: 'testFile.js',
             lineNumber: 123,
           });

packages/metro/src/lib/formatBundlingError.js

@@ -24,7 +24,6 @@ const {
 export type CustomError = Error & {|
   status?: number,
   type?: string,
-  description?: string,
   filename?: string,
   lineNumber?: number,
   errors?: Array<{
@@ -63,7 +62,7 @@ function formatBundlingError(
   ) {
     error.errors = [
       {
-        description: error.description,
+        description: error.message,
         filename: error.filename,
         lineNumber: error.lineNumber,
       },