Add missing return

Without this return statement there could appear one errorcase.
If a path is not existing it will be deleted via delete().
If all paths are gone pim will automatically cycle through them plus the initial call on a now empty path and will error out.
With the return statement the active move_index() will be finished, since delete() will call move_index() again.

(Recursion ftw)
Optim handling of corrupted images

Put everything (get path, load image, get static image(or not), zoom_factor) into try/except.
Massive try (not sure if thats wise) but else it would be necessary for an extra return.
Optimize deletion process

Problem: If at /tmp/pim-trash is already a file with that name, the actual file won't be moved to that location, but will be removed from self.paths + error message.

Solution: Iterate as long as necessary, until the file in /tmp/pim-trash can be renamed, then move the new/current file to pim-trash.
Pro:
 * No extra module
 * No name collision
 * lesser lines with a for loop
Con:
 * hardcoded to x possible duplicates
 * Dunno how much time it will need, when there is a large number of name duplicates
Check for errors in some cases:

Exchange if statements with try/except to check if something is possible or not.
This affects:
 * zoom_to
 * zoom_delta
 * rotate
 * update_image
Hide cursor on inactivity:

If the pointer won't be moved it will disappear after a given time.

Based upon a commit from rzarzynski's fork of Pim.
Marking of pictures:

Initial and primitive way to mark pictures.

Points to work on:
 * No visible indication
 * No check for duplicates
 * No way to unmark a marked picture

Further problem is: How to use the result? Piping back to pim fails (although it will work with sxiv -i) and dunno about use-cases.
Just wanted it to remove from the ToDo list.
Remove length check of path:

Path is a required option, so parsing the arguments will error out with the notice, that a path needs to supplied.
The old check isn't needed anymore.
0.6.0