HBASE-23040 give friendly message about unknown server when running unload for RegionMover.

closes apache#635

Signed-off-by: stack <stack@apache.org>

Author: busbey

hbase-server/src/main/java/org/apache/hadoop/hbase/util/RegionMover.java

@@ -421,6 +421,12 @@ public boolean unload() throws InterruptedException, ExecutionException, Timeout
                 .keySet());
         // Remove the host Region server from target Region Servers list
         ServerName server = stripServer(regionServers, hostname, port);
+        if (server == null) {
+          LOG.info("Could not find server '{}:{}' in the set of region servers. giving up.",
+              hostname, port);
+          LOG.debug("List of region servers: {}", regionServers);
+          return false;
+        }
         // Remove RS present in the exclude file
         stripExcludes(regionServers);
         stripMaster(regionServers);