Have the ronin-exploits run command automatically check if the target is exploitable, and print an error message if it's not

[postmodern]

Change the run_exploit method in the CLI::Commands::Run command to also call @exploit.perform_test and check if the return value is of type TestResult::NotVulnable and print an error message (using log_error) with the NotVulnerable message.

We may also want to add a --force option which would disable this behavior and run the exploit regardless.