Report unsupported compression in solv_xfopen() with errno

Report unsupported compression in solv_xfopen() with errno

If libsolv was built without Zstandard support and "primary.xml.zst"
was passed solv_xfopen(), solv_xfopen() returned 0 without setting
errno. A calling application could not distinguish an unsupported
compression format from other I/O errors.

This patch improves this situation by setting errno variable to an
appropriate value. The value macros are documented in POSIX 2017.