linux/storagepath: Revamp handling of user-dirs

- dont assume that ~/.config/user-dirs.dirs exists
- don't assume it does have contents or the relevant value
- don't use exception handling as happy path control flow
- when defaulting, use the path under HOME, not e.g. /Pictures

plyer/platforms/linux/storagepath.py

@@ -4,7 +4,7 @@
 '''
 
 from plyer.facades import StoragePath
-from os.path import expanduser, dirname, abspath
+from os.path import expanduser, dirname, abspath, join, exists
 
 # Default paths for each name
 USER_DIRS = "/.config/user-dirs.dirs"
@@ -22,17 +22,18 @@
 class LinuxStoragePath(StoragePath):
 
     def _get_from_user_dirs(self, name):
-        try:
-            with open(self._get_home_dir() + USER_DIRS, "r") as f:
-                lines = f.readlines()
-                # Find the line that starts with XDG_<name> to get the path
-                index = [i for i, v in enumerate(lines)
-                         if v.startswith("XDG_" + name)][0]
-                return lines[index].split('"')[1]
-        except KeyError:
-            return PATHS[name]
-        except Exception as e:
-            raise e
+        home_dir = self.__get_home_dir(),
+        default = join(home_dir, PATHS[name])
+        user_dirs = join(home_dir, USER_DIRS)
+        if not exists(user_dirs):
+            return default
+
+        with open(user_dirs, "r") as f:
+            for l in f.readlines():
+                if l.startswith("XDG_" + name):
+                    return l.split('"')[1]
+
+        return default
 
     def _get_home_dir(self):
         return expanduser('~')