Most of the LOB functions take an errorp argument, which is a boolean controlling what happens if an error occurs inside an OCI function. If errorp is true, an error is signaled. If errorp is false, the function returns an error object (of type sql-database-error).
All the LOB functions signal an error if the lob-locator argument given is not a LOB locator object as returned by select or query.
Many of the functions basically perform a call to the underlying OCI function. When the match is direct, this is mentioned in the function's manual page.