Improve error message for detecting video backend

[roomrys]

Description

This error message is very annoying because it is very uninformative:

    raise ValueError("Could not detect backend for specified filename.")
ValueError: Could not detect backend for specified filename.

This PR takes away some of the annoyance by providing the filename:

    raise ValueError(f"Could not detect backend for specified filename: {filename}")
ValueError: Could not detect backend for specified filename: D:\social-leap-estimates-animal-poses\datasets\colab.slp.training_job

Types of changes

• Bugfix
• New feature
• Refactor / Code style update (no logical changes)
• Build / CI changes
• Documentation Update
• Other (error message)

Does this address any currently open issues?

[list open issues here]

Outside contributors checklist

• Review the guidelines for contributing to this repository
• Read and sign the CLA and add yourself to the authors list
• Make sure you are making a pull request against the develop branch (not main). Also you should start your branch off develop
• Add tests that prove your fix is effective or that your feature works
• Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP!

❤️

[codecov]

Codecov Report

Merging #1441 (f979045) into develop (1151a95) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1441   +/-   ##
========================================
  Coverage    73.03%   73.03%           
========================================
  Files          133      133           
  Lines        23768    23768           
========================================
+ Hits         17358    17359    +1     
+ Misses        6410     6409    -1     

Files Changed	Coverage Δ
sleap/io/video.py	92.04% <100.00%> (+0.15%)	⬆️

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