Clearer error message if Codec loading fails under a Security Manager

[gus-asf]

Description

I just spent quite a while tracking down a problem wherein a SecurityManager was silently ignoring the META-INF/services/org.apache.lucene.codecs.Codec file. We should enhance the message in the event that a security manager is detected to alert the user to this possibility and the potentially surprising detail that no SecurityException is thrown by java when it fails to load a file via the classpath. Even more confusingly, java can load classes and then silently fail to load files from the same jar file that the class came from, so alerting the user to this complex situation seems worthwhile. (PR soon)