fix: Log errors in the background subprocess

Currently there is not output if the background subprocess exits with an error,
for example because the configuration file has a syntax error. This commit fixes
that.

Should I add tests for it?
Can you split this into multiple lines please? Something like this
```js
var backgroundArgs = {
  cmd: 'node',
  //...
};
var backgroundProcess = grunt.util.spawn(function(error) {
```