Include exception in warning message when pid file cannot be created

Closes gh-2905

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/system/ApplicationPidFileWriter.java

@@ -108,7 +108,7 @@ public class ApplicationPidFileWriter implements
 					writePidFile(event);
 				}
 				catch (Exception ex) {
-					logger.warn(String.format("Cannot create pid file %s", this.file));
+					logger.warn(String.format("Cannot create pid file %s", this.file), ex);
 				}
 			}
 		}