disc: DiscRead carries SCSI status/sense

Fix extract_scsi_context() and Error::scsi_sense() to handle Error::DiscRead
in addition to Error::ScsiError, so is_marginal_read() works for DiscRead
errors and disc::copy() can properly route MEDIUM ERROR as a marginal
(bad sector) instead of bailing.

Closes: (internal)#20260427

src/error.rs

@@ -490,6 +490,7 @@ impl Error {
     pub fn scsi_sense(&self) -> Option<&crate::scsi::ScsiSense> {
         match self {
             Error::ScsiError { sense: Some(s), .. } => Some(s),
+            Error::DiscRead { sense: Some(s), .. } => Some(s),
             _ => None,
         }
     }