Replace most subprocess.call() invocations with check_call()

[andreyv]

This ensures proper error handling. In some places manual error reporting is also replaced with Python chained exceptions.

Manual error handling is left in loaders because multiple other places also return None instead of raising an exception. It can be refactored too if needed.

---

This change is 

[andreyv]

One of the newly added checks fails:

ERROR [Worker,3 4 Worker::execute_job_group] Worker failed: Command '['isolate', '--cg', '--box-id=40', '--dir=/tmp=/tmp/cms-compile-50t1z18h/home:rw', '--run', '--', '/bin/chmod', '777', '-R', '/tmp']' returned non-zero exit status 1..

[andreyv]

The output from isolate is something like

/bin/chmod: changing permissions of '/tmp': Operation not permitted
/bin/chmod: changing permissions of '/tmp/checker': Operation not permitted
/bin/chmod: changing permissions of '/tmp/user_output.txt': Operation not permitted
/bin/chmod: changing permissions of '/tmp/input.txt': Operation not permitted
/bin/chmod: changing permissions of '/tmp/correct_output.txt': Operation not permitted
Exited with error status 1

Looking at the home directory, the mentioned files are owned by the CMS user instead of the sandbox user. The chmod command runs as the sandbox user.

Looks like this behavior is intended. A comment can be added that the exit code should be ignored.

[codecov]

Codecov Report

Merging #1128 (18959eb) into master (31599b6) will decrease coverage by 1.54%.
The diff coverage is 12.00%.

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
- Coverage   63.52%   61.98%   -1.55%     
==========================================
  Files         232      230       -2     
  Lines       17028    16611     -417     
==========================================
- Hits        10817    10296     -521     
- Misses       6211     6315     +104     

Flag	Coverage Δ
functionaltests	45.71% <12.00%> (-1.70%)	⬇️
unittests	43.25% <0.00%> (-0.88%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cms/service/PrintingService.py	0.00% <0.00%> (ø)
cmscontrib/loaders/tps.py	8.50% <0.00%> (-0.74%)	⬇️
cmstaskenv/cmsMake.py	0.00% <0.00%> (ø)
cms/grading/Sandbox.py	67.74% <75.00%> (-1.08%)	⬇️
cms/grading/languages/python3_cpython.py	42.85% <0.00%> (-57.15%)	⬇️
cmscommon/commands.py	71.42% <0.00%> (-28.58%)	⬇️
cmscommon/datetime.py	49.05% <0.00%> (-20.65%)	⬇️
cms/server/admin/handlers/contestannouncement.py	36.00% <0.00%> (-10.67%)	⬇️
cms/service/ProxyService.py	57.97% <0.00%> (-8.17%)	⬇️
cms/server/admin/handlers/contestquestion.py	26.22% <0.00%> (-7.60%)	⬇️
... and 98 more

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