fujifilm_dynamic_range: only load if exiftool present

Fail to load if exiftool binary is not present. This may be better
than silently (in UI) failing to read dyanmic range data.

Author: Dan Torop

contrib/fujifilm_dynamic_range.lua

@@ -2,7 +2,7 @@
 
 Compensate for Fujifilm raw files made using "dynamic range".
 
-Copyright (C) 2021 Dan Torop <[email protected]>
+Copyright (C) 2021, 2022 Dan Torop <[email protected]>
 
 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
@@ -116,6 +116,11 @@ local function destroy()
 	dt.destroy_event("fujifilm_dr", "post-import-image")
 end
 
+if not df.check_if_bin_exists("exiftool") then
+	dt.print_log("Please install exiftool to use fujifilm_dynamic_range")
+	error "[fujifilm_dynamic_range] exiftool not found"
+end
+
 dt.register_event("fujifilm_dr", "post-import-image", 
 	detect_dynamic_range)