Make the award level a required parameter, and improve error reporting

The award level is not an optional parameter, so don't treat it as one.
Now the command line is:

	kswatch.py [options] <project-home-url> <award-amount>

Also display the list of valid pledge levels if the given amount is not
found.

Signed-off-by: Timur Tabi <timur.tabi@calxeda.com>