nixos/test-driver: Run commands with error handling

Bash's standard behavior of not propagating non-zero exit codes
through a pipeline is unexpected and almost universally
unwanted. Default to setting `pipefail` for the command being run;
it can still be turned off by prefixing the pipeline with
`set +o pipefail` if needed.

Also, set `errexit` and `nonunset` options to make the first command
of consecutive commands separated by `;` fail, and disallow
dereferencing unset variables respectively.

nixos/lib/test-driver/test-driver.py

@@ -441,7 +441,7 @@ class Machine:
     def execute(self, command: str) -> Tuple[int, str]:
         self.connect()
 
-        out_command = "( {} ); echo '|!=EOF' $?\n".format(command)
+        out_command = "( set -euo pipefail; {} ); echo '|!=EOF' $?\n".format(command)
         self.shell.send(out_command.encode())
 
         output = ""