If concurrency >total, an error should be raised

[asaff1]

Not a very serious bug but still took me a few minutes to understand.

Command line arguments / config total = 5 connections = 1 concurrency = 6 qps = 1 format = "pretty"

Describe the bug This configuration will output a json with nulls in histogram and details. Which screws ghz-web.

Expected behavior I expect this config to raise an error to stderror, like other errors in the command line flags