add galois_init_default_field error code

add galois_init_default_field error code

galois_init_default_field returns an errno(3) code in case of error
instead of exiting. This is handy when the caller needs to perform
cleanup or error reporting when an error occurs instead of exit(2).

The exit(2) based error handling is preserved in the static
galois_init() function which is used in galois.c instead and is based on
galois_init_default_field to avoid code duplication.

Signed-off-by: Loic Dachary <loic@dachary.org>