Improve error message for PRNG seeding failure

In prng_fortuna.c, if krb5_c_random_make_octets detects that we do not
have entropy, set an error message saying that the random number
generator could not be seeded, as we likely failed previously to read
from /dev/urandom or the Windows equivalent.

https://github.com/krb5/krb5/commit/4f41a0d4a6a62439f21aacdd650595a065f15056
Author: Greg Hudson <ghudson@mit.edu>
Commit: 4f41a0d4a6a62439f21aacdd650595a065f15056
Branch: master
 src/lib/crypto/krb/prng_fortuna.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)