User token is deleted if external server cannot be reached

An error occurred when trying to contact a Crowd server to look up the user ID associated with a user token:

2017-01-17 17:59:19,293-0600 WARN [qtp1976642001-174945] *UNKNOWN com.atlassian.crowd.integration.rest.service.RestExecutor - The following URL does not specify a valid Crowd User Management REST service: https://com.server.com/crowd/rest/usermanagement/1/user?username=USERNAME
2017-01-17 17:59:19,355-0600 WARN [qtp1976642001-174945] *UNKNOWN com.sonatype.nexus.crowd.internal.CrowdUserManager - Unable to look up Crowd user USERNAME due to javax.xml.bind.DataBindingException/javax.xml.bind.UnmarshalException

• with linked exception:
  [org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Premature end of file.]
  2017-01-17 17:59:19,355-0600 DEBUG [qtp1976642001-174945] *UNKNOWN com.sonatype.nexus.usertoken.plugin.realm.UserTokenRealm - Removing stale user-token, target principals are no longer valid
  2017-01-17 17:59:19,355-0600 DEBUG [qtp1976642001-174945] *UNKNOWN com.sonatype.nexus.usertoken.plugin.internal.UserTokenServiceImpl - Removing record for: USERNAME
  2017-01-17 17:59:19,371-0600 TRACE [qtp1976642001-174945] *UNKNOWN org.sonatype.security.internal.UserIdMdcHelper - Set: USERNAME

In response to this Nexus removed the user token.

Expected:

1. Nexus should not remove the token unless the server can be reached, and the user can't be found
2. Nexus should return a 401 if the server can't be reached.