Add callback interface for errors

Printing to stderr is not the right answer for a library.

locfile.h

@@ -1,6 +1,8 @@
 #ifndef _LOCFILE_H
 #define _LOCFILE_H
 
+#include "jq.h"
+
 typedef struct {
   int start, end;
 } location;
@@ -12,9 +14,11 @@ struct locfile {
   int length;
   int* linemap;
   int nlines;
+  char *error;
+  jq_state *jq;
 };
 
-void locfile_init(struct locfile* l, const char* data, int length);
+void locfile_init(struct locfile* l, jq_state *jq, const char* data, int length);
 
 void locfile_free(struct locfile* l);