wip; Add server validation

Might need to extract a separate stateful widget.

Author: PIG208

lib/widgets/compose_box.dart

@@ -1429,12 +1429,25 @@ class _SavedSnippetComposeBoxBody extends _ComposeBoxBody {
     return Align(
       alignment: Alignment.centerRight,
       child: IconButton(
-        onPressed: () {
-          Navigator.pop(context);
-          // TODO handle validation errors; server errors
-          createSavedSnippet(store.connection,
-            title: controller.title.textNormalized,
-            content: controller.content.textNormalized);
+        onPressed: () async {
+          // TODO client-side validations
+          try {
+            await createSavedSnippet(store.connection,
+              title: controller.title.textNormalized,
+              content: controller.content.textNormalized);
+            if (!context.mounted) return;
+            Navigator.pop(context);
+          } on ApiRequestException catch (e) {
+            if (!context.mounted) return;
+            final zulipLocalizations = ZulipLocalizations.of(context);
+            final message = switch (e) {
+              ZulipApiException() => zulipLocalizations.errorServerMessage(e.message),
+              _ => e.message,
+            };
+            showErrorDialog(context: context,
+              title: zulipLocalizations.errorFailedToCreateSavedSnippet,
+              message: message);
+          }
         }, icon: Icon(ZulipIcons.check, color: designVariables.icon)));
   }
 }