Handle errors in varnishncsa formats slightly better

Print out the rest of the format, changing the error from

  "Unknown format character: {"

to:

  "Unknown format starting at: %{asdf}x"

bin/varnishncsa/varnishncsa.c

@@ -665,7 +665,7 @@ h_ncsa(void *priv, enum VSL_tag_e tag, unsigned fd,
 			/* Fall through if we haven't handled something */
 			/* FALLTHROUGH*/
 		default:
-			fprintf(stderr, "Unknown format character: %c\n", *p);
+			fprintf(stderr, "Unknown format starting at: %s\n", --p);
 			exit(1);
 		}
 	}