lower takeover leadership current log level

CHANGELOG

@@ -1,6 +1,9 @@
 devel
 -----
 
+* Lower log level to warning, when take over shard leadership finds
+  an agency Current entry is missing the server taking over.
+
 * APM-112: invalid use of OPTIONS in AQL queries will now raise a warning in
   the query. 
   The feature is useful to detect misspelled attribute names in OPTIONS, e.g.
@@ -255,8 +258,8 @@ devel
 	this way the system does only need to produce a path that is allowed to
 	be passed through.
 	e.g.
- 
-	  FOR v,e,p IN 10 OUTBOUND @start GRAPH "myGraph"
+
+    FOR v,e,p IN 10 OUTBOUND @start GRAPH "myGraph"
 		  FILTER v.isRelevant == true
 			RETURN p
 

arangod/Cluster/FollowerInfo.cpp

@@ -317,9 +317,16 @@ void FollowerInfo::takeOverLeadership(std::vector<ServerID> const& previousInsyn
         std::make_shared<std::vector<ServerID>>(previousInsyncFollowers);
     auto myEntry =
         std::find(failoverCandidates->begin(), failoverCandidates->end(), ourselves);
+
+    // We expect of course to be one of the fail over candidates. However, if the creation
+    // of the TakeOverLeadership job and it's start leading here are timewise apart, chances
+    // are that Current has been changed in the meantime. The assertion should remain here
+    // to detect any errors during CI.
     if (myEntry == failoverCandidates->end()) {
-      LOG_TOPIC("c9422", ERR, Logger::CLUSTER)
-          << "invalid failover candidates for FollowerInfo of shard " 
+      LOG_TOPIC("c9422", WARN, Logger::CLUSTER)
+          << "invalid failover candidates for FollowerInfo of shard - "
+          << "can happen, when scheduling and starting the leadership "
+          << "takeover are timewise apart, so that the Current entry has expired. "
           << _docColl->vocbase().name() << "/" << _docColl->name() 
           << ". our id: " << ourselves << ", failover candidates: "
           << *failoverCandidates << ", previous in-sync followers: "