Throw illegal arg exception if invalid input

Author: courtneyeh

validator/api/src/main/java/tech/pegasys/teku/validator/api/GraffitiManager.java

@@ -58,7 +58,7 @@ private Optional<String> updateGraffiti(final BLSPublicKey publicKey) {
   private Optional<String> updateGraffiti(final BLSPublicKey publicKey, final String graffiti) {
     final int graffitiSize = graffiti.getBytes(StandardCharsets.UTF_8).length;
     if (graffitiSize > 32) {
-      return Optional.of(
+      throw new IllegalArgumentException(
           String.format(
               "'%s' converts to %s bytes. Input must be 32 bytes or less.",
               graffiti, graffitiSize));

validator/api/src/test/java/tech/pegasys/teku/validator/api/GraffitiManagerTest.java

@@ -97,8 +97,9 @@ void setGraffiti_shouldThrowExceptionWhenGraffitiTooBig(@TempDir final Path temp
     manager = new GraffitiManager(dataDirLayout);
     assertThat(getGraffitiManagementDir().toFile().exists()).isTrue();
 
-    assertThat(manager.setGraffiti(publicKey, invalidGraffiti))
-        .hasValue(
+    assertThatThrownBy(() -> manager.setGraffiti(publicKey, invalidGraffiti))
+        .isInstanceOf(IllegalArgumentException.class)
+        .hasMessage(
             "'This graffiti is a bit too long!!' converts to 33 bytes. Input must be 32 bytes or less.");
   }