Addressing issue #3 and some TLC

[tkling]

"Real" things:

• wrapped each of the file reads in existence checks, throwing errors if required files aren't present
• addresses JSON parse exception #3 by specifically raising an error if systemKey is not present in hash that parses from /opt/jc/jcagent.conf

TLC-type things:

• removed parenthesis from parameterless method calls
• removed unneeded return statements since ruby implicitly returns the last line of a method
• removed a couple unused variables

Saw the issue on the repo and figured I could probably close it in a day :) With that in mind, these changes have been tested locally in pry and I have verified that the error handling/messaging is working as intended. I have not tested these changes on a live system to talk with Jumpcloud! None of those interactions have changed but I feel it's worth stating.

Let me know if anything in here seems undesirable, I'll change as needed :) 👍