Improve diagnostics when run does not produce expected logging

buildSrc/src/main/java/org/springframework/boot/build/docs/ApplicationRunner.java

@@ -21,6 +21,7 @@ import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -133,8 +134,10 @@ public class ApplicationRunner extends DefaultTask {
 	private void awaitLogging(Process process) {
 		long end = System.currentTimeMillis() + 30000;
 		String expectedLogging = this.expectedLogging.get();
+		List<String> outputLines = Collections.emptyList();
 		while (System.currentTimeMillis() < end) {
-			for (String line : outputLines()) {
+			outputLines = outputLines();
+			for (String line : outputLines) {
 				if (line.contains(expectedLogging)) {
 					return;
 				}
@@ -143,7 +146,10 @@ public class ApplicationRunner extends DefaultTask {
 				throw new IllegalStateException("Process exited before '" + expectedLogging + "' was logged");
 			}
 		}
-		throw new IllegalStateException("'" + expectedLogging + "' was not logged within 30 seconds");
+		StringBuilder message = new StringBuilder(
+				"After 30 seconds '" + expectedLogging + "' had not be logged in the following output:\n\n");
+		outputLines.forEach((line) -> message.append(line).append("\n"));
+		throw new IllegalStateException(message.toString());
 	}
 
 	private List<String> outputLines() {