List all trusted validator keys under single config section:

Trusted master public keys can be listed under either [validators] or
[validator_keys] config sections. All keys listed under [validators] are
added to permanent trusted keys list regardless of key type.

A master public key is moved from permanent key list to manifest cache
when one of its manifests is received. This allows rippled operators to
list all trusted keys under the [validators] config section.

src/ripple/overlay/impl/OverlayImpl.cpp

@@ -438,7 +438,8 @@ OverlayImpl::setupValidatorKeyManifests (BasicConfig const& config,
         journal_);
 
     if (!loaded)
-        Throw<std::runtime_error> ("Unable to load validator keys");
+        Throw<std::runtime_error> (
+            "Unable to load keys from [validator_keys]");
 
     auto const validation_manifest =
         config.section ("validation_manifest");