REVIEW: User ui fixes

https://issues.sonatype.org/browse/NEXUS-6807
https://issues.sonatype.org/browse/NEXUS-6808
https://issues.sonatype.org/browse/NEXUS-6825
https://issues.sonatype.org/browse/NX-703
Is it possible to have a different marker message depending whether the user is external ('You cannot update external users') vs internal ('You do not have permission to update users') ?

That would be easier for the user to understand rather than putting all possible conditions into a single message.