Feat: argument to skip frames existing in directory defined by `temp_path`

[StevenCyb]

I had a few moments when the video process was interrupted (standby, etc.). So I want to propose a flag to skip existing frames and continue the process at the last frame. This option is disabled by default.

Additionally, I moved the (in my opinion) unnecessary statement that checks after each frame if the temp folder exists. If someone deletes the folder in the meantime, the overall result is corrupt, so the program can also simply close with an error?