now missing hosts file gives nice message and no traceback

Signed-off-by: Brian Coca <briancoca+dev@gmail.com>
Merge branch 'nice_missing_inventory' of git://github.com/bcoca/ansible into devel
Slightly friendlier error on missing hosts file, slightly friendlier error on inventory script returning invalid syntax
(or if inventory is non-script and accidentally executable).
Squashed commit of the following:

    Changed ami_tags to instance_tags to better follow naming convention
    Add support for creating tags on the new instances