Surface root cause in ErrorMessage for wrapped exceptions (#2352)

* Use innermost exception message for ErrorMessage to surface root cause errors

When HttpClient wraps errors (e.g. TLS/SSL failures), the outer exception
message is generic ("An error occurred while sending the request"). Using
GetBaseException().Message surfaces the actual root cause, making diagnostics
much easier. The full exception chain remains preserved in ErrorException.

Fixes #2278

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Add tests verifying ErrorMessage surfaces innermost exception message

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: alexeyzimarev

src/RestSharp/Response/RestResponseBase.cs

@@ -161,6 +161,6 @@ protected RestResponseBase(RestRequest request) {
 
     internal void AddException(Exception exception) {
         ErrorException = exception;
-        ErrorMessage   = exception.Message;
+        ErrorMessage   = exception.GetBaseException().Message;
     }
 }

src/RestSharp/RestClient.Async.cs

@@ -65,7 +65,7 @@ static RestResponse GetErrorResponse(RestRequest request, Exception exception, C
             ResponseStatus = exception is OperationCanceledException
                 ? TimedOut() ? ResponseStatus.TimedOut : ResponseStatus.Aborted
                 : ResponseStatus.Error,
-            ErrorMessage   = exception.Message,
+            ErrorMessage   = exception.GetBaseException().Message,
             ErrorException = exception
         };
 

test/RestSharp.Tests/ErrorMessageTests.cs

@@ -0,0 +1,43 @@
+using System.Security.Authentication;
+
+namespace RestSharp.Tests;
+
+public class ErrorMessageTests {
+    [Fact]
+    public async Task ErrorMessage_surfaces_innermost_exception_message() {
+        const string innerMessage = "The remote certificate is invalid according to the validation procedure.";
+
+        var innerException = new AuthenticationException(innerMessage);
+        var wrappedException = new HttpRequestException("An error occurred while sending the request.", innerException);
+
+        var handler = new FakeHandler(wrappedException);
+        var client = new RestClient(new RestClientOptions("https://dummy.org") { ConfigureMessageHandler = _ => handler });
+
+        var response = await client.ExecuteAsync(new RestRequest("/"));
+
+        response.ErrorMessage.Should().Be(innerMessage);
+        response.ErrorException.Should().BeOfType<HttpRequestException>();
+        response.ErrorException!.InnerException.Should().BeOfType<AuthenticationException>();
+        response.ResponseStatus.Should().Be(ResponseStatus.Error);
+    }
+
+    [Fact]
+    public async Task ErrorMessage_uses_direct_message_when_no_inner_exception() {
+        const string message = "No such host is known.";
+
+        var exception = new HttpRequestException(message);
+        var handler = new FakeHandler(exception);
+        var client = new RestClient(new RestClientOptions("https://dummy.org") { ConfigureMessageHandler = _ => handler });
+
+        var response = await client.ExecuteAsync(new RestRequest("/"));
+
+        response.ErrorMessage.Should().Be(message);
+        response.ErrorException.Should().BeOfType<HttpRequestException>();
+        response.ErrorException!.InnerException.Should().BeNull();
+    }
+
+    class FakeHandler(Exception exception) : HttpMessageHandler {
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+            => throw exception;
+    }
+}