CffMapping: ignore invalid yaml files

For example:

• https://sentry.softwareheritage.org/share/issue/7398cf7d0f6b40b1b24ffcf7e180328e/ (cause)
• https://sentry.softwareheritage.org/share/issue/9d26a1ae0e7d440aa226066782029945/ (cause)

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Migrated from T4276 (view on Phabricator)