fix: set nonzero exit code for SIGTERMException (#21623)

* fix: set nonzero exit code for SIGTERMException (#19916)

`SIGTERMException(SystemExit)` was raised without arguments, causing
`SystemExit.code` to default to `None`, which Python converts to exit
code 0. Scripts monitoring the training process exit code could not
distinguish a SIGTERM termination from a successful exit.

Override `__init__` to pass 143 (128 + SIGTERM signal number 15) to
`SystemExit`, following the standard Unix convention.

Fixes #19916

* Apply suggestion from @deependujha

* Apply suggestion from @deependujha

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* Update src/lightning/pytorch/utilities/exceptions.py

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Co-authored-by: Deependu <deependujha21@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>