misc: option_parser: streamline #[source] and Error impl

This streamlines the Error implementation in the Cloud Hypervisor code
base to match the remaining parts so that everything follows the agreed
conventions. These are leftovers missed in the previous commits.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

option_parser/src/lib.rs

@@ -155,7 +155,9 @@ impl OptionParser {
 
 pub struct Toggle(pub bool);
 
+#[derive(Error, Debug)]
 pub enum ToggleParseError {
+    #[error("invalid value: {0}")]
     InvalidValue(String),
 }
 
@@ -176,8 +178,9 @@ impl FromStr for Toggle {
 
 pub struct ByteSized(pub u64);
 
-#[derive(Debug)]
+#[derive(Error, Debug)]
 pub enum ByteSizedParseError {
+    #[error("invalid value: {0}")]
     InvalidValue(String),
 }
 
@@ -207,7 +210,9 @@ impl FromStr for ByteSized {
 
 pub struct IntegerList(pub Vec<u64>);
 
+#[derive(Error, Debug)]
 pub enum IntegerListParseError {
+    #[error("invalid value: {0}")]
     InvalidValue(String),
 }
 
@@ -297,11 +302,16 @@ impl TupleValue for Vec<usize> {
 
 pub struct Tuple<S, T>(pub Vec<(S, T)>);
 
+#[derive(Error, Debug)]
 pub enum TupleError {
+    #[error("invalid value: {0}")]
     InvalidValue(String),
-    SplitOutsideBrackets(OptionParserError),
-    InvalidIntegerList(IntegerListParseError),
-    InvalidInteger(ParseIntError),
+    #[error("split outside brackets: {0}")]
+    SplitOutsideBrackets(#[source] OptionParserError),
+    #[error("invalid integer list: {0}")]
+    InvalidIntegerList(#[source] IntegerListParseError),
+    #[error("invalid integer: {0}")]
+    InvalidInteger(#[source] ParseIntError),
 }
 
 impl<S: FromStr, T: TupleValue> FromStr for Tuple<S, T> {
@@ -339,7 +349,9 @@ impl<S: FromStr, T: TupleValue> FromStr for Tuple<S, T> {
 #[derive(Default)]
 pub struct StringList(pub Vec<String>);
 
+#[derive(Error, Debug)]
 pub enum StringListParseError {
+    #[error("invalid value: {0}")]
     InvalidValue(String),
 }