#394 Cleanup rspec output

[pshelby]

Closes #394

Description

Currently when running all rspec tests for cfn_nag, there is close to 300 lines of output written to the screen (see Before section below). This makes it difficult when debugging new rules. By adding error types where needed and suppressing stderr/stdout from cfn_nag_executor calls, this PR will cleanup all erroneous rspec output.

Testing

1. All rspec tests executed successfully before and after the changes.

Before

rspec_output.txt

After

cfn_nag_dev@caefac1f2dd9:/workspaces/cfn_nag$ bundle exec rspec
Run options: exclude {:end_to_end=>true}
................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
.........................................................................................................................

Finished in 8.43 seconds (files took 2.78 seconds to load)
697 examples, 0 failures

Coverage report generated for RSpec to /workspaces/cfn_nag/coverage. 2737 / 2826 LOC (96.85%) covered.