Show stacktrace of startup exception

Whether or not the stacktrace is displayed is controlled by bootstrap
log level setting, so that bootstrap: DEBUG displays the stack trace on
output, like it does on log

Closes #5102

src/main/java/org/elasticsearch/ExceptionsHelper.java

@@ -23,6 +23,9 @@
 import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.rest.RestStatus;
 
+import java.io.PrintWriter;
+import java.io.StringWriter;
+
 /**
  *
  */
@@ -110,4 +113,11 @@ public static String detailedMessage(Throwable t, boolean newLines, int initialC
             return t.getClass().getSimpleName() + "[" + t.getMessage() + "]";
         }
     }
+
+    public static String stackTrace(Throwable e) {
+        StringWriter stackTraceStringWriter = new StringWriter();
+        PrintWriter printWriter = new PrintWriter(stackTraceStringWriter);
+        e.printStackTrace(printWriter);
+        return stackTraceStringWriter.toString();
+    }
 }

src/main/java/org/elasticsearch/bootstrap/Bootstrap.java

@@ -274,6 +274,9 @@ private static String buildErrorMessage(String stage, Throwable e) {
         } else {
             errorMessage.append("- ").append(ExceptionsHelper.detailedMessage(e, true, 0));
         }
+        if (Loggers.getLogger(Bootstrap.class).isDebugEnabled()) {
+            errorMessage.append("\n").append(ExceptionsHelper.stackTrace(e));
+        }
         return errorMessage.toString();
     }
 }