More verbose error message for invalid JSON in config.js

if you enter an invalid json object such as:
```
{
server: "http://foo.com"
host: "0.0.0.0"
}
```
while configuring, you get errors but are still able to run forever and the message previously was not as indicative of other potential configuration problems.
Merge  pull request #1225 from enahs/patch-1