Fix UnicodeEncodeError exception handling (#160)

When build_cache() encounters a UnicodeEncodeError, the original code
did not escape unprintable chars and caused the cache build to fail.
This patch fixes the code so that the culprit is correctly printed in
debug mode.

src/dmenu_extended/main.py

@@ -786,11 +786,11 @@ def cache_save(self, items, path):
             for item in items:
                 clean = True
                 for char in item:
-                    if char not in string.printable:
+                    if not str.isprintable(char):
                         clean = False
                         foundError = True
                         if self.debug:
-                            print("Culprit: " + item)
+                            print("Culprit: " + item.encode('unicode_escape').decode("utf-8"))
                 if clean:
                     tmp.append(item)
             if foundError:
@@ -802,7 +802,7 @@ def cache_save(self, items, path):
                     print("Offending items have been excluded from cache")
                 with open(path, "wb") as f:
                     for item in tmp:
-                        f.write(item + "\n")
+                        f.write(item.encode('unicode_escape') + b"\n")
                 return 2
             else:
                 if self.debug: