Rename CanThisReallyOccur to InvalidSliceLength

The CanThisReallyOccur error variant can really occur, namely if the
slice length is invalid, i. e. either the encoded data length Lc is
wrong, or the length values Lc and Le are not encoded properly.
Therefore, this patch renames the CanThisReallyOccur variant to a more
informative name.

src/command.rs

@@ -74,7 +74,7 @@ pub enum FromSliceError {
     TooShort,
     InvalidClass,
     InvalidFirstBodyByteForExtended,
-    CanThisReallyOccur,
+    InvalidSliceLength,
 }
 
 impl From<class::InvalidClass> for FromSliceError {
@@ -216,7 +216,11 @@ fn parse_lengths(body: &[u8]) -> Result<ParsedLengths, FromSliceError> {
         return Ok(parsed);
     }
 
-    Err(FromSliceError::CanThisReallyOccur)
+    // If we haven’t returned yet, the slice has an invalid length:  Either the encoded lc value is
+    // wrong, or the lc and le lengths are not encoded properly (one byte per value for simple
+    // APDU, two bytes per value for extended APDU).
+
+    Err(FromSliceError::InvalidSliceLength)
 }
 
 #[cfg(test)]