[bugfix] SIGTERM and SIGQUIT handling

[gsmolk]

Currently, SIGTERM and SIGQUIT are not handled in any meaningful way.

======================================================================
FAIL: test_sigterm_hadnling (tests.backup_test.BackupTest)
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Traceback (most recent call last):
  File "tests/backup_test.py", line 1162, in test_sigterm_hadnling
    'Backup STATUS should be "ERROR"')
AssertionError: Backup STATUS should be "ERROR"

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Ran 1 test in 2.328s

FAILED (failures=1)

I think, it should be handled exactly as SIGINT.

[gsmolk]

Fixed by @alubennikova