[server] Fix server considering ClientExceptions as successes

Miscategorized ClientExceptions as successes. Moved the success return statement to prevent such cases in the future. Added a test dedicated to the ClientException failure mode (test existed for 500s).

server/lib/aquamarine_server.dart

@@ -299,6 +299,7 @@ class AquamarineServer {
             final uv = await ofsClient.fetchUv(simulationTime);
             await persistence.writeUv(simulationTime, latlngHash!, uv);
           }
+          return FetchResult.success;
         } on ResourceException catch (e) {
           log.warning(e.message);
 
@@ -312,8 +313,8 @@ class AquamarineServer {
           }
         } on http.ClientException catch (e, s) {
           log.warning('Failed to fetch $simulationTime', e, s);
+          return FetchResult.transientFailure;
         }
-        return FetchResult.success;
       });
 
       switch (result) {

server/test/aquamarine_server_test.dart

@@ -9,6 +9,7 @@ import 'package:aquamarine_server_interface/io.dart';
 import 'package:aquamarine_server_interface/types.dart';
 import 'package:clock/clock.dart';
 import 'package:file/memory.dart';
+import 'package:http/http.dart' as http;
 import 'package:latlng/latlng.dart';
 import 'package:mockito/annotations.dart';
 import 'package:mockito/mockito.dart';
@@ -397,7 +398,7 @@ void main() {
     );
 
     test(
-      'continues trying in the case of transient failures',
+      'continues trying in the case of transient failures with status codes',
       () => withClock(clock, () async {
         final ofsClient = MockOfsClient();
         final persistence = MockPersistence();
@@ -419,6 +420,30 @@ void main() {
       }),
     );
 
+    test(
+      'continues trying in the case of transient failures with client '
+      'exceptions',
+      () => withClock(clock, () async {
+        final ofsClient = MockOfsClient();
+        final persistence = MockPersistence();
+
+        final t = simulationTime.timestamp;
+
+        final server = AquamarineServer(
+          ofsClient: ofsClient,
+          persistence: persistence,
+        );
+
+        when(ofsClient.fetchLatLngUv(any))
+            .thenAnswer((_) async => throw http.ClientException('foo'));
+
+        expect(
+            await server.fetchSimulationRun(t), FetchResult.transientFailure);
+
+        verify(ofsClient.fetchLatLngUv(any)).called(6 + 1 + 48);
+      }),
+    );
+
     test(
       'skips up-to-date hours',
       () => withClock(clock, () async {