Verify that GLES3 cached shader load is successful

[ChibiDenDen]

Fixes #77136

Verified by corrupting the shader data using
variant_bytes.ptrw()[5] = 0;
right before loading.
Shader cache loading fails but the shader compilation kicks in and everything still works

[clayjohn]

Looks great! This is exactly what I had in mind. Tested locally by switching between GPUs and can confirm this fixes the issue

[akien-mga]

Thanks!