fix(decode_backtrace): handle non-exist debug info better

[fruch]

#10646 introduced some unsafe code

the when debug info doesn't it fail like the following:

20:58:29      raise Exception("Couldn't find scylla debug information")
20:58:29  Exception: Couldn't find scylla debug information
20:58:29
20:58:29  During handling of the above exception, another exception occurred:
20:58:29
20:58:29  Traceback (most recent call last):
20:58:29    File "/usr/local/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
20:58:29      self.run()
20:58:29    File "/usr/local/lib/python3.10/threading.py", line 953, in run
20:58:29      self._target(*self._args, **self._kwargs)
20:58:29    File "/home/ubuntu/scylla-cluster-tests/sdcm/cluster.py", line 1600, in decode_backtrace
20:58:29      if "is closed" in details:
20:58:29  TypeError: argument of type 'Exception' is not iterable

the mistake was to treat Exception as string, with turn it into a string

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

[soyacz]

LGTM