sbt: Getting error output from an external process (yarn)

We run into a problem with 'yarn build' command run from build.sbt:

println(Process("yarn build", frontEndFile).!!)

The output is just:

[info] yarn build 3456
[error] java.lang.RuntimeException: Nonzero exit value: 1

But we don't see the acutal error output from yarn. How can we get that error output from the yarn process?

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