rust/pv_core: Fix the argument order in 'LengthMismatch'

Fix the ordering of the parameters in 'TryFrom<Confidential<Vec<u8> for
Confidential<[u8; N>'. While at it, convert 'LengthMismatch' error to a
named struct.

Fixes: 7608cf2de4 ("rust/confidential: Add `From` and `Into` for confidential byes arrays/vectors")
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

rust/pv_core/src/error.rs

@@ -82,8 +82,8 @@ pub enum Error {
     #[error("Cannot decode hex string")]
     InvHexStringChar { source: std::num::ParseIntError },
 
-    #[error("Expected size {0}, found {1}")]
-    LengthMismatch(usize, usize),
+    #[error("Expected size {expected}, actual {actual}")]
+    LengthMismatch { expected: usize, actual: usize },
 }
 
 /// Error cases for I/O operations