HBASE-19755 Error message for non-existent namespace is inaccurate

Signed-off-by: tedyu <yuzhihong@gmail.com>

hbase-shell/src/main/ruby/shell/commands.rb

@@ -117,7 +117,7 @@ module Shell
           raise "Unknown region #{args.first}!"
         end
         if cause.is_a?(org.apache.hadoop.hbase.NamespaceNotFoundException)
-          raise "Unknown namespace #{args.first}!"
+          raise "Unknown namespace #{args.first.split(':')[0]}!"
         end
         if cause.is_a?(org.apache.hadoop.hbase.snapshot.SnapshotDoesNotExistException)
           raise "Unknown snapshot #{args.first}!"