Cli: Improve output for usage errors

When a cli throws a USAGE error, it is implied that the user did
something wrong, and probably needs help in understanding the cli
arguments. This change adds help output before the usage error is
printed.

core/src/main/java/org/elasticsearch/cli/Command.java

@@ -56,6 +56,9 @@ public abstract class Command {
             terminal.println(Terminal.Verbosity.SILENT, "ERROR: " + e.getMessage());
             return ExitCodes.USAGE;
         } catch (UserError e) {
+            if (e.exitCode == ExitCodes.USAGE) {
+                printHelp(terminal);
+            }
             terminal.println(Terminal.Verbosity.SILENT, "ERROR: " + e.getMessage());
             return e.exitCode;
         }

core/src/test/java/org/elasticsearch/cli/CommandTests.java

@@ -34,6 +34,16 @@ public class CommandTests extends ESTestCase {
         }
     }
 
+    static class UsageErrorCommand extends Command {
+        UsageErrorCommand() {
+            super("Throws a usage error");
+        }
+        @Override
+        protected void execute(Terminal terminal, OptionSet options) throws Exception {
+            throw new UserError(ExitCodes.USAGE, "something was no good");
+        }
+    }
+
     static class NoopCommand extends Command {
         boolean executed = false;
         NoopCommand() {
@@ -120,4 +130,15 @@ public class CommandTests extends ESTestCase {
         assertEquals(output, ExitCodes.DATA_ERROR, status);
         assertTrue(output, output.contains("ERROR: Bad input"));
     }
+
+    public void testUsageError() throws Exception {
+        MockTerminal terminal = new MockTerminal();
+        UsageErrorCommand command = new UsageErrorCommand();
+        String[] args = {};
+        int status = command.main(args, terminal);
+        String output = terminal.getOutput();
+        assertEquals(output, ExitCodes.USAGE, status);
+        assertTrue(output, output.contains("Throws a usage error"));
+        assertTrue(output, output.contains("ERROR: something was no good"));
+    }
 }