Abort requests when SSE headers cannot be set (#2806) AddSseRequestHead() returned true even when the SSE-C key lookup failed, and all eight callers logged a warning but continued without encryption. If something goes wrong with SSE setup, data would be sent unencrypted. In practice the error path is unreachable today because startup validation in FinalCheckSse() prevents s3fs from running with SSE-C and an empty key list. But the code should not rely on that -- if AddSseRequestHead fails, the request must not proceed. Return false from AddSseRequestHead on SSE-C key failure and abort with -EIO at all call sites, consistent with how PreHeadRequest already handles this case.