Add exception handle for illegal arg

Author: courtneyeh

Diff for: validator/api/src/main/java/tech/pegasys/teku/validator/api/GraffitiManager.java

@@ -91,7 +91,7 @@ public Optional<Bytes32> getGraffitiFromStorage(final BLSPublicKey publicKey) {
     final Path filePath = graffitiPath.get().resolve(resolveFileName(publicKey));
     try {
       return Optional.of(GraffitiParser.loadFromFile(filePath)).filter(this::graffitiNotEmpty);
-    } catch (GraffitiLoaderException e) {
+    } catch (GraffitiLoaderException | IllegalArgumentException e) {
       LOG.error("Unable to read graffiti from storage.", e);
       return Optional.empty();
     }