feat(motion_velocity_smoother): add a diagnostic for smoothing failure

[mkuri]

Description

Fixed a problem with slow deceleration when rapid deceleration is required

Related links

https://tier4.atlassian.net/browse/RT0-24965

Tests performed

1. Spawn obstacles in front of the vehicle to rapid deceleration
2. The diagnostics of "smoother_errors" is triggered and emergency stop is performed.

Notes for reviewers

Interface changes

Effects on system behavior

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[0x126]

LGTM