Better error message if a user doesn't specify any arguments

[ssokolow]

ssokolow@monolith rdbackup-excludes-indexer [master] %% cargo profiler     
error: Invalid profiler. cargo profiler currently supports callgrind and cachegrind.

Referring to "no profiler" as an invalid profiler is one of those things that only makes sense to a computer.

The error message should be one of the following in this case:

• error: You must specify a profiler.
• USAGE: cargo profiler <SUBCOMMAND>
• The output of cargo profiler --help

Also, I noticed that your actual USAGE: doesn't use < and > to specify that the subcommand is mandatory.

I can't double-check, since I get an error in rustc-serialize when I attempt to build your git HEAD, but I think you can just add .required(true) to the subcommand argument to get a more helpful error message automatically, plus the appropriate <SUBCOMMAND> in your --help rather than [SUBCOMMAND].

(I'm not 100% certain because I use subcommands infrequently enough in my projects that the last time I defined one was back when clap didn't exist and I was working with argparse and Python.)