[Review] Request from 'digitaltom' @ 'SUSE/connect/review_140813_small_connect_improvements_that_came_out_of_card_2149'

Please review the following changes:
  * 9de507a cache output of 'uname -i' as it certainly won't change during the run of SUSEconnect
  * 2b7f027 improve error message when instancedata not found
  * d481c34 correctly catch invalid cmd options, fix error exit codes
```ruby
unless File.readable?(file_path) should be enough
```