Suggestion: unknown command - respond with enhanced metadata in exception

[mgravell]

For example, someone tries to run HSETEX on a v6 server; we could have the library report that this is a server 8.0 feature, in a single consistent unified location.

Notes:

• do not change .Message - just the .ToString(), so that existing consumers are not adversely impacted

[mgravell]

suggestion via @NickCraver when discussing the hash field API extensions; suggest we push this back to the work to refactor the IO core, as there will already be a need to revisit most messages