JRuby: Better Handling of Agent JSON Parsing Errors

If the `javaagent.json` file is not successfully parsed, [this line](https://github.com/appneta/oboe-ruby/blob/master/lib/joboe_metal.rb#L75) fails on stack boot.  Although this is an error condition, the gem shouldn't crash (validate this) and instead just go into no-op mode with a logged informational warning.