Show help at start of command

[HuwCampbell]

It's pretty hard at the moment to show the full help text just at the start of a command or sub command, e.g., if I type git or git add (which both fail as they have missing things) it should look like git --help or git add --help respectively, while other parse errors only show the minimum.

Exit code should still be a failure however, and if this is on we should probably also suppress the Missing: blah error information (as it's redundant).

commercialhaskell/stack#809

[HuwCampbell]

#207