Is your feature request related to a problem? Please describe.
Improving the frame extraction interface in the GUI and make it more user friendly.

Describe the solution you'd like
When the user wants to extract frames manually it is necessary to select a video. Currently the video selection has the title: Optional frame extraction from a video subset. While this title is informative is not linked to the manual extraction.
For example: Frame Extraction from a Video Subset (Mandatory Step for Manual Extraction)

In edition when the user tries to extract frames manually without choosing a video first there is an error that is not clear
'Traceback (most recent call last):
File "/Users/tomdeadman/miniforge3/envs/DEEPLABCUT_M1/lib/python3.9/site-packages/deeplabcut/gui/tabs/extract_frames.py", line 197, in extract_frames
_ = launch_napari(list(self.video_selection_widget.files)[0])
IndexError: list index out of range'

We could add some information. An example:
No video file selected. Please upload a video to proceed with manual frame extraction.