feat: make Error implement std::fmt::Display, std::error::Error` and Sync (#47)

This is necessary to make it work with error-wrapping libraries like
[anyhow](https://crates.io/crates/anyhow) or
[eyre](https://crates.io/crates/eyre). 

Also remove Error::Unexpected as it appeared unused.

Author: hkratz

eventsource-client/src/error.rs

@@ -6,11 +6,11 @@ pub enum Error {
     TimedOut,
     StreamClosed,
     /// An invalid request parameter
-    InvalidParameter(Box<dyn std::error::Error + Send + 'static>),
+    InvalidParameter(Box<dyn std::error::Error + Send + Sync + 'static>),
     /// The HTTP response could not be handled.
     UnexpectedResponse(StatusCode),
     /// An error reading from the HTTP response body.
-    HttpStream(Box<dyn std::error::Error + Send + 'static>),
+    HttpStream(Box<dyn std::error::Error + Send + Sync + 'static>),
     /// The HTTP response stream ended
     Eof,
     /// The HTTP response stream ended unexpectedly (e.g. in the
@@ -20,13 +20,32 @@ pub enum Error {
     InvalidLine(String),
     InvalidEvent,
     /// Encountered a malformed Location header.
-    MalformedLocationHeader(Box<dyn std::error::Error + Send + 'static>),
+    MalformedLocationHeader(Box<dyn std::error::Error + Send + Sync + 'static>),
     /// Reached maximum redirect limit after encountering Location headers.
     MaxRedirectLimitReached(u32),
-    /// An unexpected failure occurred.
-    Unexpected(Box<dyn std::error::Error + Send + 'static>),
 }
 
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        use Error::*;
+        match self {
+            TimedOut => write!(f, "timed out"),
+            StreamClosed => write!(f, "stream closed"),
+            InvalidParameter(err) => write!(f, "invalid parameter: {err}"),
+            UnexpectedResponse(status_code) => write!(f, "unexpected response: {status_code}"),
+            HttpStream(err) => write!(f, "http error: {err}"),
+            Eof => write!(f, "eof"),
+            UnexpectedEof => write!(f, "unexpected eof"),
+            InvalidLine(line) => write!(f, "invalid line: {line}"),
+            InvalidEvent => write!(f, "invalid event"),
+            MalformedLocationHeader(err) => write!(f, "malformed header: {err}"),
+            MaxRedirectLimitReached(limit) => write!(f, "maximum redirect limit reached: {limit}"),
+        }
+    }
+}
+
+impl std::error::Error for Error {}
+
 impl PartialEq<Error> for Error {
     fn eq(&self, other: &Error) -> bool {
         use Error::*;
@@ -50,19 +69,9 @@ impl Error {
     pub fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self {
             Error::HttpStream(err) => Some(err.as_ref()),
-            Error::Unexpected(err) => Some(err.as_ref()),
             _ => None,
         }
     }
 }
 
-impl<E> From<E> for Error
-where
-    E: std::error::Error + Send + 'static,
-{
-    fn from(e: E) -> Error {
-        Error::Unexpected(Box::new(e))
-    }
-}
-
 pub type Result<T> = std::result::Result<T, Error>;