redis-benchmark silently fails on unknown Redis commands

[itamarhaber]

For example:

$ redis-benchmark -n 1 -c 1 foo
====== foo ======
  1 requests completed in 0.00 seconds
  1 parallel clients
  3 bytes payload
  keep alive: 1

100.00% <= 0 milliseconds
inf requests per second

Note - also tested with other non-commands, with the same results:

- $ redis-benchmark -n 1 -c 1 foo
+ $ redis-benchmark -n 1 -c 1 moo

[filipecosta90]

@itamarhaber closing this one given it was fixed by #8869 =)