[postprocessor:ugoira] improve error messages (#406)

gallery_dl/postprocessor/ugoira.py

@@ -103,6 +103,7 @@ class UgoiraPP(PostProcessor):
 
             # invoke ffmpeg
             pathfmt.set_extension(self.extension)
+            try:
                 if self.twopass:
                     if "-f" not in args:
                         args += ["-f", self.extension]
@@ -112,7 +113,12 @@ class UgoiraPP(PostProcessor):
                 else:
                     args.append(pathfmt.realpath)
                     self._exec(args)
-
+            except OSError as exc:
+                print()
+                self.log.error("Unable to invoke FFmpeg (%s: %s)",
+                               exc.__class__.__name__, exc)
+                pathfmt.realpath = pathfmt.temppath
+            else:
                 if self.delete:
                     pathfmt.delete = True
                 else: