upstream: silence (to log level debug2) failure messages when

loading the default hostkeys. Hostkeys explicitly specified in the
configuration or on the command-line are still reported as errors, and
failure to load at least one host key remains a fatal error.
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Based on patch from Dag-Erling Smørgrav via
https://github.com/openssh/openssh-portable/pull/103

ok markus@

OpenBSD-Commit-ID: ffc2e35a75d1008effaf05a5e27425041c27b684
