Animation import bug fixes

A few bug fixes to the animation import.

• Fix crash on importing unsupported video format
• Fix animation import preview when less than 1 sec
• Try to better guess import animation fps

Doing some tests, it seems GIFs made in photoshop are not showing proper fps. This patch tries retreive a more accurate fps.

Test Plan

1. Import an AV1 video on an FFMpeg version that doesn't support it, it should just give an error and not crash
2. Import a GIF that is less than 1 second long, and move the preview bar, it should now properly show the frames
3. Import a GIF that is made in photoshop, it should show proper FPS instead of 100 FPS

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.