Add more error explanations in the BMP image loader

[Calinou]

This closes #32166 and closes #30629.

[akien-mga]

modules/bmp/image_loader_bmp.cpp:55:117: error: conversion from 'size_t' (aka 'unsigned long') to 'const Variant' is ambiguous
                                        vformat("1-bpp BMP images must have a width that is a multiple of 8, but the imported BMP is %d pixels wide.", width));
                                                                                                                                                       ^~~~~

You should probably cast to int (or uint64_t).

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.3.