misc: api_client: streamline #[source] and Error

This streamlines the code base to follow best practices for
error handling in Rust: Each error struct implements
std::error::Error (most due via thiserror::Error derive macro)
and sets its source accordingly.

This allows future work that nicely prints the error chains,
for example.

So far, the convention is that each error prints its
sub error as part of its Display::fmt() impl.

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

api_client/src/lib.rs

@@ -12,15 +12,15 @@ use vmm_sys_util::sock_ctrl_msg::ScmSocket;
 #[derive(Debug, Error)]
 pub enum Error {
     #[error("Error writing to or reading from HTTP socket: {0}")]
-    Socket(std::io::Error),
+    Socket(#[source] std::io::Error),
     #[error("Error sending file descriptors: {0}")]
-    SocketSendFds(vmm_sys_util::errno::Error),
+    SocketSendFds(#[source] vmm_sys_util::errno::Error),
     #[error("Error parsing HTTP status code: {0}")]
-    StatusCodeParsing(std::num::ParseIntError),
+    StatusCodeParsing(#[source] std::num::ParseIntError),
     #[error("HTTP output is missing protocol statement")]
     MissingProtocol,
     #[error("Error parsing HTTP Content-Length field: {0}")]
-    ContentLengthParsing(std::num::ParseIntError),
+    ContentLengthParsing(#[source] std::num::ParseIntError),
     #[error("Server responded with an error: {0:?}: {1:?}")]
     ServerResponse(StatusCode, Option<String>),
 }