Fix #419, improve Cannot index message (sortof)

jv_get() doesn't know if it's being called in the context of a jq
program or not, so it can't produce a very useful error message when the
types of the to-be-indexed value and the key don't agree.  For now,
including the key (when it is a short string) in the error message is as
significant an improvement as is easy to make.

jv_aux.c

@@ -95,9 +95,20 @@ jv jv_get(jv t, jv k) {
     jv_free(k);
     v = jv_null();
   } else {
-    v = jv_invalid_with_msg(jv_string_fmt("Cannot index %s with %s",
-                                          jv_kind_name(jv_get_kind(t)),
-                                          jv_kind_name(jv_get_kind(k))));
+    /*
+     * If k is a short string it's probably from a jq .foo expression or
+     * similar, in which case putting it in the invalid msg may help the
+     * user.  The length 30 is arbitrary.
+     */
+    if (jv_get_kind(k) == JV_KIND_STRING && jv_string_length_bytes(jv_copy(k)) < 30) {
+      v = jv_invalid_with_msg(jv_string_fmt("Cannot index %s with string \"%s\"",
+                                            jv_kind_name(jv_get_kind(t)),
+                                            jv_string_value(k)));
+    } else {
+      v = jv_invalid_with_msg(jv_string_fmt("Cannot index %s with %s",
+                                            jv_kind_name(jv_get_kind(t)),
+                                            jv_kind_name(jv_get_kind(k))));
+    }
     jv_free(t);
     jv_free(k);
   }