Raise error instead of exiting

Exiting with specific codes will be handled over in CLI.
Handle errors and exits in CLI.
Remove expectation on error message

This message is present in the exception that the class #find method can
throw. We shouldn't have expectations for the exceptions since they'll
be handled higher up.
Include name of bad algorithm in exception message
Don't exit in other places than CLI

Raise errors instead and let CLI handle it.
Remove usage of the have_error_message matcher

It's a bit over-the-top to have "ERROR" displayed, and it's not used
enough to warrant its existence.
Minor tweak to error output
Merge pull request #26 from linduxed/move_error_and_exit_handling_to_cli

Move error handling into CLI class
Split feature spec into two
Add acceptance test for length argument