More descriptive error message for BBH headon

[JosePineda684]

Proposed changes

The following change addresses issue #6240 with a more clarifying error message in the event someone attempts to start a bbh headon collision with both orbital_angular_velocity=0 and eccentricity=0.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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• The code follows the stylistic and code quality guidelines listed in the
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• The PR lists upgrade instructions and is labeled bugfix or
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Further comments