Improve handling of sfLedgerSequence field:

The sfLedgerSequence field is designated as optional in the object
template but it is effectively required and validations which do not
include it were, correctly, rejected.

This commit migrates the check outside of the peer code and into the
constructor used for validations being deserialized for the network.

Furthermore, the code will generate an error if a validation that is
generated by a server does not include the field.

src/ripple/overlay/impl/PeerImp.cpp

@@ -2170,7 +2170,8 @@ PeerImp::onMessage(std::shared_ptr<protocol::TMValidation> const& m)
     }
     catch (std::exception const& e)
     {
-        JLOG(p_journal_.warn()) << "Validation: Exception, " << e.what();
+        JLOG(p_journal_.warn())
+            << "Exception processing validation: " << e.what();
         fee_ = Resource::feeInvalidRequest;
     }
 }