Add Exception to model Elasticsearch exceptions (#118)

This commit adds an Exception struct that models
exceptions returned by Elasticsearch in the event of an error.

An exception() function is exposed on Response to deserialize the
response body into Exception when the status code is
in the 400-599 range.

Consideration was made to exposing this on
the Error struct, performing the deserialization inside
`error_for_status_code`, but this introduces some awkwardness into
the API:

- `error_for_status_code` would ideally be async because the .json()
  call is async
- .json() consumes the response, which would not be possible for
  `error_for_status_code_ref` which has a shared reference to Response.

Consideration was made to combining `Error` and `Cause` into one type, since 
they both map to `ElasticsearchException` on the server side. It looks like 
certain properties are only ever exposed on the top level exception however, 
which is deserialized to `Error` e.g. 
root_cause, header and failed_shards (which is not currently mapped in Error).
In light of this, decided to keep them separate for now.

Additional details about an exception are collected in a map and exposed
through additional_details function.

Closes #112

Author: russcam

elasticsearch/Cargo.toml

@@ -31,6 +31,7 @@ url = "^2.1"
 serde = { version = "~1", features = ["derive"] }
 serde_json = "~1"
 serde_with = "~1"
+void = "1.0.2"
 
 [dev-dependencies]
 chrono = { version = "^0.4", features = ["serde"] }

elasticsearch/examples/search_questions_answers/main.rs

@@ -31,7 +31,6 @@ use std::env;
 use sysinfo::SystemExt;
 use url::Url;
 mod stack_overflow;
-use elasticsearch::http::response::Response;
 use stack_overflow::*;
 use textwrap::fill;
 

elasticsearch/src/cert.rs

@@ -16,6 +16,7 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+//! Certificate components
 
 use crate::error::Error;
 use std::{
@@ -229,8 +230,8 @@ impl Certificate {
         }
 
         if certs.is_empty() {
-            Err(Error::lib(
-                "could not find PEM certificate in input data".to_string(),
+            Err(crate::error::lib(
+                "could not find PEM certificate in input data",
             ))
         } else {
             Ok(Self(certs))

elasticsearch/src/error.rs

@@ -32,13 +32,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-use crate::http::transport::BuildError;
+use crate::http::{transport::BuildError, StatusCode};
 use std::{error, fmt, io};
 
-/// An error within the client.
+/// An error with the client.
 ///
 /// Errors that can occur include IO and parsing errors, as well as specific
-/// errors from Elasticsearch and internal errors from this library
+/// errors from Elasticsearch and internal errors from the client.
 #[derive(Debug)]
 pub struct Error {
     kind: Kind,
@@ -102,10 +102,34 @@ impl From<BuildError> for Error {
     }
 }
 
+pub(crate) fn lib(err: impl Into<String>) -> Error {
+    Error {
+        kind: Kind::Lib(err.into()),
+    }
+}
+
 impl Error {
-    pub(crate) fn lib(err: impl Into<String>) -> Self {
-        Error {
-            kind: Kind::Lib(err.into()),
+    /// The status code, if the error was generated from a response
+    pub fn status_code(&self) -> Option<StatusCode> {
+        match &self.kind {
+            Kind::Http(err) => err.status(),
+            _ => None,
+        }
+    }
+
+    /// Returns true if the error is related to a timeout
+    pub fn is_timeout(&self) -> bool {
+        match &self.kind {
+            Kind::Http(err) => err.is_timeout(),
+            _ => false,
+        }
+    }
+
+    /// Returns true if the error is related to serialization or deserialization
+    pub fn is_json(&self) -> bool {
+        match &self.kind {
+            Kind::Json(_) => true,
+            _ => false,
         }
     }
 }