script/ptl-tool: handle cherry-pick conflicts on branch-specific commits

script/ptl-tool: handle cherry-pick conflicts on branch-specific commits

Previously, ConflictSimulationCheck only anticipated merge conflicts
when simulating upstream cherry-picks. If a branch-specific commit (such
as test repo branch adjustments or backport metadata changes) failed to
apply cleanly to the target base branch, the script threw an unhandled
GitCommandError and crashed with an unhandled traceback.

Wrap branch-specific commit cherry-picks in a try...except block. When a
conflict occurs, abort the cherry-pick, record a "Rebase Required"
simulation failure in the audit report, and either prompt the user in
interactive mode or gracefully halt check execution in CI mode.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>