Additional help messages

[natecook1000]

This adds some new messages to error and help output that guide users to find more information.

Validation errors now include a message about using --help:

$ roll --bad-flag
Error: Unknown option '--bad-flag'
Usage: roll [--times <n>] [--sides <m>] [--seed <seed>] [--verbose]
  See 'roll --help' for more information.

The help screen now shows a message about the implicit help subcommand when the help screen contains subcommands:

$ math --help
OVERVIEW: A utility for performing maths.

USAGE: math <subcommand>

OPTIONS:
  --version               Show the version.
  -h, --help              Show help information.

SUBCOMMANDS:
  add                     Print the sum of the values.
  multiply                Print the product of the values.
  stats                   Calculate descriptive statistics.

  See 'math help <subcommand>' for detailed help.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test