Reduce log level of handled AD group membership traversal exception

[cmshawns]

The following exception doesn't occur very often, and doesn't appear to actually impact the user's experience (observed in the Class Schedule).

I suspect this is a resurfaced issue we saw early on - where this error occurred while traversing a group membership tree that contained a group from a different domain.

This is currently being logged as an Error, but should really be a Warn, as it does not need to be e-mailed for immediate attention.

Unable to query Active Directory for user 'XXXXXXXXXXXXXX': System.Runtime.InteropServices.COMException (0x80005000): Unknown error (0x80005000)
   at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
   at System.DirectoryServices.DirectoryEntry.Bind()
   at System.DirectoryServices.DirectoryEntry.get_AdsObject()
   at System.DirectoryServices.DirectorySearcher.FindAll(Boolean findMoreThanOne)
   at System.DirectoryServices.AccountManagement.ADDNLinkedAttrSet.GetNextSearchResult()
   at System.DirectoryServices.AccountManagement.ADDNLinkedAttrSet.MoveNextMemberSearcher()
   at System.DirectoryServices.AccountManagement.ADDNLinkedAttrSet.MoveNext()
   at System.DirectoryServices.AccountManagement.FindResultEnumerator`1.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at CtcApi.Web.Security.ActiveDirectoryRoleProvider.RecurseGroup(PrincipalContext context, String group, List`1 groups)
   at CtcApi.Web.Security.ActiveDirectoryRoleProvider.GetRolesForUser(String username)