sql: cluster setting errors should list valid values

[BramGruneir]

This would be incredibly useful for enum settings. e.g.

root@:26257/defaultdb> set cluster setting server.remote_debugging.mode = always;
pq: invalid mode: 'always'
root@:26257/defaultdb> set cluster setting server.remote_debugging.mode = on;
pq: invalid mode: 'on'
root@:26257/defaultdb> set cluster setting server.remote_debugging.mode = true;
pq: argument of SET CLUSTER SETTING server.remote_debugging.mode must be type string, not type bool
root@:26257/defaultdb> set cluster setting server.remote_debugging.mode = 'true';
invalid syntax: statement ignored: driver: bad connection
warning: connection lost!
opening new connection: all session settings will be lost
warning: error retrieving the transaction status: dial tcp [::1]:26257: connect: connection refused
warning: connection lost!
opening new connection: all session settings will be lost
warning: error retrieving the database name: dial tcp [::1]:26257: connect: connection refused
root@:26257/? ?> set cluster setting server.remote_debugging.mode = 'true';
warning: connection lost!
opening new connection: all session settings will be lost
pq: invalid mode: 'true'
root@:26257/defaultdb> set cluster setting server.remote_debugging.mode = 'true';
pq: invalid mode: 'true'
root@:26257/defaultdb> set cluster setting server.remote_debugging.mode = 'always';
pq: invalid mode: 'always'
root@:26257/defaultdb> show cluster setting server.remote_debuggin.mode;
pq: unknown setting: "server.remote_debuggin.mode"
root@:26257/defaultdb> show cluster setting server.remote_debugging.mode;
  server.remote_debugging.mode

@knz for triage

[jordanlewis]

@BramGruneir this would probably take you 10 minutes to implement - want to take care of it real quick?