Fail a CPU blit whose source format cannot be converted

[f1nalspace]

When surface_cpu_blt() needs a format conversion and surface_convert_format() has no
converter for it, it logs "Cannot convert source surface format to dest format." and jumps
to release: without setting hr, which is still WINED3D_OK from its initialization.
The caller reports success although nothing was copied.

This sets WINED3DERR_NOTAVAILABLE at that point, like the other unsupported cases in the
same function.

Found while tracking down black DXT1 textures in Descent 3 (see the DXT1 PR). On its own it
does not change what that game shows — Descent 3 ignores the return value of Blt() — but
an application that checks it now sees the failure.

Tested: built on main with the MinGW cross toolchain. Not run on its own.