Log a warning in case of missing sts dependency.

Fixes aws#1915.

Author: ascheja

core/auth/src/main/java/software/amazon/awssdk/auth/credentials/internal/WebIdentityCredentialsUtils.java

@@ -19,12 +19,14 @@
 import software.amazon.awssdk.annotations.SdkInternalApi;
 import software.amazon.awssdk.auth.credentials.WebIdentityTokenCredentialsProviderFactory;
 import software.amazon.awssdk.core.internal.util.ClassLoaderHelper;
+import software.amazon.awssdk.utils.Logger;
 
 /**
  * Utility class used to configure credential providers based on JWT web identity tokens.
  */
 @SdkInternalApi
 public final class WebIdentityCredentialsUtils {
+    private static final Logger log = Logger.loggerFor(WebIdentityCredentialsUtils.class);
 
     private static final String STS_WEB_IDENTITY_CREDENTIALS_PROVIDER_FACTORY =
         "software.amazon.awssdk.services.sts.internal.StsWebIdentityCredentialsProviderFactory";
@@ -44,7 +46,9 @@ public static WebIdentityTokenCredentialsProviderFactory factory() {
                     WebIdentityCredentialsUtils.class);
             return (WebIdentityTokenCredentialsProviderFactory) stsCredentialsProviderFactory.getConstructor().newInstance();
         } catch (ClassNotFoundException e) {
-            throw new IllegalStateException("To use web identity tokens, the 'sts' service module must be on the class path.", e);
+            String message = "To use web identity tokens, the 'sts' service module must be on the class path.";
+            log.warn(() -> message);
+            throw new IllegalStateException(message, e);
         } catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) {
             throw new IllegalStateException("Failed to create a web identity token credentials provider.", e);
         }