Throwing an error within a plugin does not stop the build

# Details

When creating a plugin, throwing an error inside it won't stop the build, it will just cause it to timeout.

# How to reproduce

I didn't know how to create a repro on the playground for a plugin, so I am pasting the code of the plugin here:

```
import { Plugin, PluginBuild } from 'esbuild';

export const moduleExportsPlugin = (): Plugin => ({
name: 'module-exports',
setup(build: PluginBuild) {
build.onEnd(_result => {
throw new Error('module.exports already exists, unknown use case');
});
},
});
```

# Versions

It seems a regression, in v0.16.x It worked fine, since 0.17.x it causes the unexpected behaviour