Conversion service does not validate that the underlying file cache entry is present

Conversion service does not validate that the underlying file cache entry is present.  Conversion fails without any exception, warning, or error until the caller attempts to open the file stream for the blob.

When the result is retrieved from the cache, the FileBlob is handed back to the caller without any validation that the underlying file is still present:

https://github.com/nuxeo/nuxeo/blob/4e351e1830fc0dbe1ba19a2eb071aefba92c9113/nuxeo-core/nuxeo-core-convert/src/main/java/org/nuxeo/ecm/core/convert/service/ConversionServiceImpl.java#L335

The construction of the file blob does not guarantee existence of the file:

https://github.com/nuxeo/nuxeo/blob/4e351e1830fc0dbe1ba19a2eb071aefba92c9113/nuxeo-core/nuxeo-core-convert/src/main/java/org/nuxeo/ecm/core/convert/cache/SimpleCachableBlobHolder.java#L62

Something else is removing the converter cache entry, either by TTL or size.