Send command output as log in tfs build email

I have a build that running some npm tests with TFS 2017. I set an email notification for failed builds.

Developers asks to get more information about the npm test instead of just "npm failed with exit code 1" so I want to get the entire output as a log file attached to the email or parse the output and send it as the error message to the email notification.

I tried to modify the BuildCompletedEvent.xsl to get different value but I keep getting the uninformative npm failure message.

Does someone know of a way to send the entire command output to the email or something similar?

via chen