avoid a runtime error when calling ra_system:overview/1 on non started systems

[flaviogrossi]

Proposed Changes

Calling ra_system:overview/1 on a non started system causes a runtime error instead of returning system_not_started as per the spec. This simple change should fix that.

Types of Changes

What types of changes does your code introduce to this project? Put an x in the boxes that apply

• [x] Bug fix (non-breaking change which fixes issue #NNNN)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation (correction or otherwise)
• [ ] Cosmetics (whitespace, appearance)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask on the mailing list. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I have read the CONTRIBUTING.md document
• [x] I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• [x] All tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)
• [x] Any dependent changes have been merged and published in related repositories

[michaelklishin]

Thank you!