[minor] Clarified error message returned when a config file contains invalid JSON.
[minor] Clarified error message returned when a config file contains invalid JSON.

remy/nconf,indexzero/nconf,HansHammel/nconf,bryce-gibson/nconf,NickHeiner/nconf,Dependencies/nconf,olalonde/nconf,ChristianMurphy/nconf,philip1986/nconf,imjerrybao/nconf