fix: give detailed error message when ps is not installed (#482)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Added a command-line check to ensure the availability of the `ps`
command before executing the main logic of the status check, enhancing
system stability and preventing runtime errors.
- **Bug Fixes**
- Improved error handling by outputting a descriptive message when the
`ps` command is not found, allowing users to understand the issue
clearly.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

Author: njzjz

dpdispatcher/machines/shell.py

@@ -74,6 +74,7 @@ def check_status(self, job):
 
         # mark defunct process as terminated
         ret, stdin, stdout, stderr = self.context.block_call(
+            r"""command -v ps >/dev/null 2>&1 || { echo >&2 "I require ps but it's not installed. Aborting."; exit 1; };"""
             f"if ps -p {job_id} > /dev/null && ! (ps -o command -p {job_id} | grep defunct >/dev/null) ; then echo 1; fi"
         )
         if ret != 0: