UnhandledPromiseRejectionWarning: FirebaseError: Task

### Environment info

**firebase-tools:**
Version: 7.3.2

**Platform:**
macOS: 10.14.6
### Test case
I tried to upload files to firebase hosting with `firebase deploy --only hosting` which throws error.

### Steps to reproduce

I tried to upload files to firebase hosting with `firebase deploy --only hosting`. In my files I have symlinks, **one of the symlink points to a file that doesn't exist**. This causes error.
### Expected behavior

Upload files without any issues.

### Actual behavior
Throws UnhandledPromiseRejectionWarning: FirebaseError: Task

```
=== Deploying to 'project'...

i deploying hosting
i hosting[project]: beginning deploy...
i hosting[project]: found 141 files in client/build

Error: Task scripts/symlink-file.js.map failed: retries exhausted after 1 attempts
(node:56592) UnhandledPromiseRejectionWarning: FirebaseError: Task scripts/symlink-file.js.map failed: retries exhausted after 1 attempts
at Queue. (/Users/user/.nvm/versions/node/v12.9.1/lib/node_modules/firebase-tools/lib/throttler/throttler.js:189:27)
at Generator.next ()
at /Users/user/.nvm/versions/node/v12.9.1/lib/node_modules/firebase-tools/lib/throttler/throttler.js:7:71
at new Promise ()
at __awaiter (/Users/user/.nvm/versions/node/v12.9.1/lib/node_modules/firebase-tools/lib/throttler/throttler.js:3:12)
at Queue.executeTask (/Users/user/.nvm/versions/node/v12.9.1/lib/node_modules/firebase-tools/lib/throttler/throttler.js:180:16)
at Queue. (/Users/user/.nvm/versions/node/v12.9.1/lib/node_modules/firebase-tools/lib/throttler/throttler.js:93:36)
at Generator.next ()
at /Users/user/.nvm/versions/node/v12.9.1/lib/node_modules/firebase-tools/lib/throttler/throttler.js:7:71
at new Promise ()
(node:56592) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 3)
(node:56592) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
```