Include path in error message in path_to_beam_jar

Right now, the error message looks like this when the job server jar can't be found:

12:35:50 RuntimeError: Please build the server with
12:35:50 cd /home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python37_PR/src; ./gradlew runners:flink:1.9:job-server:shadowJar

I would like to know the path of the missing jar to help me debug.