Convert av error numbers to friendly error messages

[IanButterworth]

The error seen here is difficult to figure out because we don't auto convert the error number to a meaningful message, which I believe should be straightforward

https://discourse.julialang.org/t/videoio-avformat-open-input-error-code-5/81758

[mrufsvold]

Hi! I second this proposal. I'm new to the package and tried to following the tutorial on an .mp4 on my computer and got avformat_open_input error code -2. I'm not sure how to approach a next step. Converted errors would be very helpful!