Loadso's Unload returns bool to indicate success/failure

[CowKeyMan]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

While debugging loading dynamic libraries, and in this case, while debugging unloading them, it would be good to have the necessary debug tools. This PR allows the user to rule out any possibility that it is loadso's unloading that is failing, but rather something else in the user's code. Or if it actually is failing, then the user will know of the failure and also get an appropriate error message from the OS.

Existing Issue(s)

No existing issues

[slouken]

I'll throw this in the 4.x milestone, since changing a function's return value changes the ABI.