Get rid of the global error pointer and replace it with a local one that gets passed to functions manually

[FSMaxB]

The new safe functions, see #58, should get a parameter for the error pointer. That would be a pointer to a pointer of char.

[DaveGamble]

cJSON_ParseWithOpts implements this. 2nd parameter is ptr to use for ep. If not provided it uses global ep. You should probably just hide ep for v2.

[FSMaxB]

Yes indeed.