Added check if the PXF PID exists before trying a start/restart in PXF bash script.

[hpandeycodeit]

This is related to github issue https://github.com/greenplum-db/sre-test/issues/274

In case the PXF pid doesn't exists, the PXF specific error to be thrown and not the OS one.

Currently if the pid doesn't exists, the command ps -p "" runs which returns error: list of process IDs must follow -p which is not very useful.

[hpandeycodeit]

We need more discussion around this as to find a better way to handle the stale PID. For now we can put this PR on hold.

# Currently, in case of any error if PXF doesn't come up, the empty PID file created here remains.
# If the pxf is restarted again after the failure, this empty PID file will cause issues. For e.g.
# isRunning function will try to ps on the empty PID which will fail with the error "error: list of process IDs must follow -p"
# To avoid that we can either
# 1. Check if the PID exists in the isRunning function
# 2. Removes the PID file which is created here after checking for write permissions
# 3. Touch a file with another name to check for the write permissions in the directory
# 4. In case PXF doesn't come up for any reason, clean the stale PID