Symlinks cause silent errors on push

This is not the same as #255 or #264, although definitely related.

Bad symlinks will cause `clasp push` to fail - this is good. What's bad is the given error message. Luckily I solved it pretty quickly, but I if I were having a bad day I could have been here a while.

I can successfully perform a push by fixing any bad symlinks in my project. What's important here is the error message returned by clasp.

## Expected Behavior

After `clasp push`, the reported error should be informative.

## Actual Behavior

```sh
$ clasp push
Push failed. Errors:
````

## Steps to Reproduce the Problem

1. Create a new project: `clasp create`
1. Create a bad symlink, `ln -s /this/does/not/exist.ts test.ts`
1. Call `clasp push`

## Specifications

- Node version: 8.12
- Version: 2.01
- OS: Linux - Ubuntu 18.04