catch errors when encoding entity map

Summary:
We've had several reports of people's content failing to save in our product. The only error message we have is this (with no callstack, unfortunately):
```
Unknown DraftEntity key: undefined.
```
I poked around in the DraftJS code, and my guess is that this is due to some character having an entity key associated with it that for whatever reason does not appear in the entity map. This diff "fixes" the issue by not throwing an error in this situation.

This is definitely not as good as figuring out the root cause of the entity mismatch, but it seems reasonable for `convertFromDraftStateToRaw` to work with slightly malformed input.

Reviewed By: niveditc

Differential Revision: D16421104

fbshipit-source-id: a2046ad0cb166dac97187fd3c524f9c8cb4017ea

src/model/encoding/__tests__/__snapshots__/convertFromDraftStateToRaw-test.js.snap

@@ -1,5 +1,28 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`must be able to convert from draft state entities not in map 1`] = `
+Object {
+  "blocks": Array [
+    Object {
+      "data": Object {},
+      "depth": 0,
+      "entityRanges": Array [
+        Object {
+          "key": 0,
+          "length": 7,
+          "offset": 0,
+        },
+      ],
+      "inlineStyleRanges": Array [],
+      "key": "a",
+      "text": "badlink",
+      "type": "unstyled",
+    },
+  ],
+  "entityMap": Object {},
+}
+`;
+
 exports[`must be able to convert from draft state with ContentBlock to raw 1`] = `
 Object {
   "blocks": Array [

src/model/encoding/__tests__/convertFromDraftStateToRaw-test.js

@@ -61,8 +61,8 @@ const treeContentState = contentState.set(
   ]),
 );
 
-const getMetadata = entityKey =>
-  Immutable.Repeat(CharacterMetadata.create({entity: entityKey}), 5);
+const getMetadata = (entityKey, length) =>
+  Immutable.Repeat(CharacterMetadata.create({entity: entityKey}), length);
 const getLink = entityKey =>
   new DraftEntityInstance({
     type: 'LINK',
@@ -79,23 +79,23 @@ const contentStateWithNonContiguousEntities = ContentState.createFromBlockArray(
       key: 'a',
       type: 'unstyled',
       text: 'link2 link2 link3',
-      characterList: getMetadata('2')
+      characterList: getMetadata('2', 5)
         .toList()
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('2'))
+        .concat(getMetadata('2', 5))
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('3')),
+        .concat(getMetadata('3', 5)),
     }),
     new ContentBlock({
       key: 'b',
       type: 'unstyled',
       text: 'link4 link2 link5',
-      characterList: getMetadata('4')
+      characterList: getMetadata('4', 5)
         .toList()
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('2'))
+        .concat(getMetadata('2', 5))
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('5')),
+        .concat(getMetadata('5', 5)),
     }),
   ],
 )
@@ -104,6 +104,15 @@ const contentStateWithNonContiguousEntities = ContentState.createFromBlockArray(
   .addEntity(getLink('4'))
   .addEntity(getLink('5'));
 
+const contentStateWithEntitiesNotInMap = ContentState.createFromBlockArray([
+  new ContentBlock({
+    key: 'a',
+    type: 'unstyled',
+    text: 'badlink',
+    characterList: getMetadata('999', 7).toList(),
+  }),
+]);
+
 const assertConvertFromDraftStateToRaw = content => {
   expect(convertFromDraftStateToRaw(content)).toMatchSnapshot();
 };
@@ -119,3 +128,7 @@ test('must be able to convert from draft state with ContentBlockNode to raw', ()
 test('must be able to convert from draft state with noncontiguous entities to raw', () => {
   assertConvertFromDraftStateToRaw(contentStateWithNonContiguousEntities);
 });
+
+test('must be able to convert from draft state entities not in map', () => {
+  assertConvertFromDraftStateToRaw(contentStateWithEntitiesNotInMap);
+});

src/model/encoding/convertFromDraftStateToRaw.js

@@ -117,12 +117,17 @@ const encodeRawEntityMap = (
   const rawEntityMap = {};
 
   Object.keys(entityMap).forEach((key, index) => {
-    const entity = contentState.getEntity(DraftStringKey.unstringify(key));
-    rawEntityMap[index] = {
-      type: entity.getType(),
-      mutability: entity.getMutability(),
-      data: entity.getData(),
-    };
+    try {
+      const entity = contentState.getEntity(DraftStringKey.unstringify(key));
+      rawEntityMap[index] = {
+        type: entity.getType(),
+        mutability: entity.getMutability(),
+        data: entity.getData(),
+      };
+    } catch {
+      // keep contructing rawEntityMap even if one entity referenced in
+      // character metadata doesn't exist in contentState
+    }
   });
 
   return {