Watch crashes after a syntax error when running BTR with blocks

**Bug**

When a syntax error occurs in a module that is executed in BTR and uses Blocks the watch build crashes and is not recoverable.

```
UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 6): 
Error: ENOENT: no such file or directory, stat '../output/dev/src/widgets/Home.js'
```