host: Unlock write mutex when attach fails
host: Unlock write mutex when attach fails

If this mutex doesn’t get unlocked, the error write will hang trying to
lock at L175.

In practice this was happening when getting the logs for interactive and
non-existent jobs.

Closes #444

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

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