Add exit code check to workflow that tests with soroban-examples diff step (#1596)

### What
Add exit code validation to the diff step in the soroban-examples
workflow.

  ### Why
The workflow expects a diff to exist after regeneration. Without exit
code checking, the step passes even when no diff is present, masking if
the workflow stops working at patching the sdk.

Author: leighmcculloch

.github/workflows/test-with-soroban-examples.yml

@@ -80,7 +80,7 @@ jobs:
         done
 
     - name: Diff
-      run: git diff
+      run: (! git diff --exit-code) || (echo 'A diff is expected'; exit 1)
 
     - name: Build soroban-examples
       env: