[8.0] fix SSHCE: clearer error message when output is not found

[aldbr]

Currently, when the SSHCE fails to get the output, we get the following error:

2024-06-06T11:44:40,902705Z WorkloadManagement/SiteDirectorDteam/ce.ce.ce ERROR: Invalid output from remote command
Traceback (most recent call last):
  File "/opt/dirac/versions/v9.0.0a29-1717652456/Linux-x86_64/lib/python3.11/site-packages/DIRAC/Resources/Computing/SSHComputingElement.py", line 511, in __executeHostCommand
    index = output.index("============= Start output ===============")
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: substring not found

The message is quite misleading.

• I added a condition where output is empty
• I removed the exception stack trace and the log when it is invalid

The SiteDirector is expected to print the S_ERROR message.

BEGINRELEASENOTES
*WorkloadManagement
FIX: remove exception log when output is not found
ENDRELEASENOTES

[DIRACGridBot]

Sweep summary

Sweep ran in https://github.com/DIRACGrid/DIRAC/actions/runs/9420068098

Successful:

• integration