Enhance _exec_cmd function to improve error handling and output collection

[axellpadilla]

fixes #35

[flarco]

thanks @axellpadilla , I checked out the branch on my local machine and ran python -m pytest tests/tests.py -v, got this error:

$ python -m pytest tests/tests.py -v
=============================================================================== test session starts ================================================================================
platform darwin -- Python 3.11.8, pytest-8.3.4, pluggy-1.5.0 -- /Users/fritz/.local/share/virtualenvs/sling-python-G4ZuZLzI/bin/python
cachedir: .pytest_cache
rootdir: /Users/fritz/__/Git/sling-python/sling
plugins: anyio-4.3.0
collected 0 items / 1 error                                                                                                                                                        

====================================================================================== ERRORS ======================================================================================
_________________________________________________________________________ ERROR collecting tests/tests.py __________________________________________________________________________
../../../../.local/share/virtualenvs/sling-python-G4ZuZLzI/lib/python3.11/site-packages/_pytest/python.py:493: in importtestmodule
    mod = import_path(
../../../../.local/share/virtualenvs/sling-python-G4ZuZLzI/lib/python3.11/site-packages/_pytest/pathlib.py:587: in import_path
    importlib.import_module(module_name)
/opt/homebrew/Cellar/python@3.11/3.11.8/Frameworks/Python.framework/Versions/3.11/lib/python3.11/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1204: in _gcd_import
    ???
<frozen importlib._bootstrap>:1176: in _find_and_load
    ???
<frozen importlib._bootstrap>:1147: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:690: in _load_unlocked
    ???
../../../../.local/share/virtualenvs/sling-python-G4ZuZLzI/lib/python3.11/site-packages/_pytest/assertion/rewrite.py:184: in exec_module
    exec(co, module.__dict__)
tests/tests.py:6: in <module>
    from sling import (
E     File "/Users/fritz/__/Git/sling-python/sling/sling/__init__.py", line 527
E       raise Exception(f"Sling command failed:\n{'\n'.join(lines)}")
E                                                                   ^
E   SyntaxError: f-string expression part cannot include a backslash
============================================================================= short test summary info ==============================================================================
ERROR tests/tests.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
================================================================================= 1 error in 0.17s =================================================================================

[axellpadilla]

@flarco , got it, python 3.11 doesn't support it, I tested with python 3.9, 3.10, 3.11 and fixed compatibility now

[flarco]

Thanks 👍