Merge pull request #22777 from davidbilge

* pr/22777:
  Polish "Propagate the cause that led Reactor"s debug agent to fail"
  Propagate the cause that led Reactor"s debug agent to fail

Closes gh-22777

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/reactor/DebugAgentEnvironmentPostProcessor.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -49,7 +49,7 @@ public class DebugAgentEnvironmentPostProcessor implements EnvironmentPostProces
 					debugAgent.getMethod("init").invoke(null);
 				}
 				catch (Exception ex) {
-					throw new RuntimeException("Failed to init Reactor's debug agent");
+					throw new RuntimeException("Failed to init Reactor's debug agent", ex);
 				}
 			}
 		}