[postprocessor:exec] add a better error message (#155)

gallery_dl/job.py

@@ -330,7 +330,7 @@ class DownloadJob(Job):
                     pp_obj = pp_cls(self.pathfmt, pp_dict)
                 except Exception as exc:
                     postprocessor.log.error(
-                        "%s: initialization failed: %s %s",
+                        "'%s' initialization failed:  %s: %s",
                         name, exc.__class__.__name__, exc)
                 else:
                     self.postprocessors.append(pp_obj)

gallery_dl/postprocessor/exec.py

@@ -16,7 +16,13 @@ class ExecPP(PostProcessor):
 
     def __init__(self, pathfmt, options):
         PostProcessor.__init__(self)
-        self.args = options["command"]
+
+        try:
+            self.args = options["command"]
+            self.args[0]  # test if 'args' is subscriptable
+        except (KeyError, IndexError, TypeError):
+            raise TypeError("option 'command' must be a non-empty list")
+
         if options.get("async", False):
             self._exec = subprocess.Popen