Add usage output, other clean up

[notlesh]

Adds usage output (e.g. --help) and improves error handling. I also intend to support runtime-wide benchmarks in this PR.

Current usage:

Benchbot usage:

/bench --help
        print this help output
/bench <SUBCOMMAND>
        run the given subcommand

SUBCOMMAND:
    pallet <pallet>
            benchmark a specific pallet and update its weights.rs file
    runtime
            benchmark an entire runtime and update all pallet weights.rs files
            (currently only supports moonbase runtime)