Fix two problems with the display of help messages:

1. If an operation has a required option, then passing only a 'help'
    argument to it without any options will cause the program to simply
    print the usage(ie, cli form and options), but it will not show the
    detailed help pages.

2. If a required option is not supplied then the program will not tell you which required option is missing, but it will only show the usage.

Oh, also that if one just run 'aws' (no arguments) then no usage will be printed. This patch also fixes that.