Only try read `resp` if there was no error

[andyw8]

Fixes #790

NOT YET TESTED

Warn Checklist

• [ ] This changes the interface and requires a Major/Minor version change.
• [ ] I have :tophat:'d these changes by using the commands I changed by hand.
• [ ] I have added a dependancy to the project.
• [ ] I have considered any potential impact on node-themekit

[tanema]

I have taken care of this in #809 because the problem was a lot larger than this.