JSONParser needs a little bit more context when reporting an error

Occasionally a Key/Value Pair evaluates to an empty string. In that case, the error message is something like:

Caused by: java.lang.IllegalArgumentException: Malformatted JSON key-value pair: 
	at org.apache.felix.serializer.impl.json.JSONParser.parseKeyValue(JSONParser.java:106)
	at org.apache.felix.serializer.impl.json.JSONParser.parseObject(JSONParser.java:203)
	... 56 more

This makes it very challenging to find where the error is in the file.

In order to add more context, the previously-parsed Key/Value pair should also be output.