Improve exceptions on link level errors

Pass original exception also if http request fails with error status. Dio throws DioError if response status is in error range so this makes it possible to have that as original error.

GraphQL is agnostic about how transport is handled including errors so many server implementations return always 200 OK even for error responses instead of using status codes like 400.
Still, depending on your server solution, authentication errors might be returned with error status 401 with non-graphql formatted body (e.g. plain string).
So therefore there might not be any parsedResponse and the default toString of the link error gives you nothing. So this change includes toString override for the DioLinkServerException that should give more useful output out-of-the-box.

Author: jamonkko

links/gql_dio_link/lib/src/dio_link.dart

@@ -164,7 +164,10 @@ class DioLink extends Link {
                 ? parser.parseResponse(res.data as Map<String, dynamic>)
                 : null;
             throw DioLinkServerException(
-                response: res, parsedResponse: parsedResponse);
+              response: res, 
+              parsedResponse: parsedResponse,
+              originalException: e
+            );
           }
         case dio.DioErrorType.DEFAULT:
         default:

links/gql_dio_link/lib/src/exceptions.dart

@@ -24,12 +24,24 @@ class DioLinkServerException extends ServerException {
   /// Response which caused the exception
   final dio.Response response;
 
-  const DioLinkServerException({
-    @required this.response,
-    @required Response parsedResponse,
-  }) : super(
-          parsedResponse: parsedResponse,
-        );
+  const DioLinkServerException(
+      {@required this.response,
+      @required Response parsedResponse,
+      dynamic originalException})
+      : super(
+            parsedResponse: parsedResponse,
+            originalException: originalException);
+
+  @override
+  String toString() {
+    final dynamic data = response.data;
+    String plainResponse = "...";
+    if (data is String) {
+      plainResponse =
+          data.length > 80 ? data.replaceRange(80, data.length, "...") : data;
+    }
+    return "DioLinkServerException(originalException: $originalException, status: ${response.statusCode}, response: ${parsedResponse ?? plainResponse}";
+  }
 }
 
 @immutable

links/gql_dio_link/test/gql_dio_link_test.dart

@@ -483,6 +483,62 @@ void main() {
           ),
         ),
       );
+
+      // Dio returned the failed response instead of throwing, so there is no exception
+      expect(exception.originalException, null);
+    });
+
+    test("throws DioLinkServerException for dio error response", () async {
+      final error = dio.DioError(
+          response:
+              dio.Response<String>(data: "Not authenticated", statusCode: 401),
+          type: dio.DioErrorType.RESPONSE);
+
+      when(
+        client.post<dynamic>(
+          any,
+          data: anyNamed("data"),
+          options: anyNamed("options"),
+        ),
+      ).thenThrow(
+        error,
+      );
+
+      DioLinkServerException exception;
+
+      try {
+        await execute().first;
+      } catch (e) {
+        exception = e as DioLinkServerException;
+      }
+
+      expect(
+        exception,
+        TypeMatcher<DioLinkServerException>(),
+      );
+      expect(
+        exception.response.data,
+        error.response.data,
+      );
+      expect(
+        exception.response.statusCode,
+        error.response.statusCode,
+      );
+      // Failed to parse non-grahpql formatted body, so therefore null
+      expect(
+        exception.parsedResponse,
+        null,
+      );
+
+      expect(exception.originalException != null, true);
+
+      final dioException = exception.originalException as dio.DioError;
+      expect(dioException.response.data, "Not authenticated");
+      expect(dioException.response.statusCode, 401);
+      expect(dioException.type, dio.DioErrorType.RESPONSE);
+
+      expect(exception.toString(),
+          "DioLinkServerException(originalException: DioError [DioErrorType.RESPONSE]: , status: 401, response: Not authenticated");
     });
 
     test("throws HttpLinkServerException when no data and errors", () async {