util/u_format: don't crash in util_format_translate if we can't do translation

util/u_format: don't crash in util_format_translate if we can't do translation

Some formats can't be handled - in particular cannot handle ints/uints formats,
which lack the pack_rgba_float/unpack_rgba_float functions. Instead of trying
to call these (and crash) return an error (I'm not sure yet if we should try
to translate such formats too here might not make much sense).

v2: suggested by Jose, use separate checks for pack/unpack of rgba_8unorm and
rgba_float functions (right now if one exists the other should as well).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>