Fix reporter output when running mocha from grunt-exec

Error details from spec reporter are not displayed on the console when running mocha from grunt-exec task.
Using a grunt-exec configuration like:
```
exec:{
  tests:{
    command:'node ./node_modules/mocha/bin/mocha ./tests --recursive --reporter spec'
  }
}
```
The output stops after ">>   14 failing", the error details for each failing test and the number of tests passing are missing.