Invalid Access Log Path Crashes memcached

Some customers like to set the access log path to a custom location. If this path does not exist, memcached will throw an unhandled exception trying to generate the access log.

Backtrace shows the AccessScanner's visitor task calls open on the MutationLog without handing the read exception.

Recreated on 4.6.3 by setting the access log path as follows:

curl -i -u Administrator:password --data 'ns_bucket:update_bucket_props("default", [{extra_config_string, "alog_path=/mnt/haiko/does_not_exist"}]).' http://localhost:8091/diag/eval

/etc/init.d/couchbase-server restart

/opt/couchbase/bin/cbepctl localhost:11210 -b default set flush_param alog_sleep_time 2

Log from repro:
https://s3.amazonaws.com/cb-engineering/davidH/collectinfo-2018-01-17T142039-ns_1%4010.112.163.101.zip