error message related to problems with pre/post_observation_script should be more explicit

While testing the new satnogs-client version 1.0 in a raspberry pi 4 with buster, I setup the pre_observation_script and post_observation_script values. I got exceptions when running the observation. The cause was not immediate to see, the path was just wrong. And later on, when the path was fixed, I still got the same error. This time, I decided to add the line #!/usr/bin/bash on top, and well, the error disappeared.

I would like the error message in this part of the code be a bit more explicit with these user errors/problems.