Improve error message for FIXED config options

[FlorianHockmann]

Describe the feature: When someone tries to change a config option which has the mutability level FIXED, then this error message is logged:

Local setting graph.set-vertex-id=true (Type: FIXED) is overridden by globally managed value (false). Use the ManagementSystem interface instead of the local configuration to control this setting

However, FIXED options cannot be changed at all in an existing cluster as the docs state:

FIXED: Like GLOBAL, but the value cannot be changed once the JanusGraph cluster is initialized.

It would be good to state this clearly in the error message instead of suggesting that users try the ManagementSystem instead which won't work.

This was just described on Discord and StackOverflow.