Improve error message when no ServerProperties bean is found

Closes gh-11511

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerPropertiesAutoConfiguration.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -29,7 +29,6 @@ import org.springframework.context.ApplicationContextAware;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.Ordered;
-import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
 /**
@@ -81,9 +80,14 @@ public class ServerPropertiesAutoConfiguration {
 			// a single bean
 			String[] serverPropertiesBeans = this.applicationContext
 					.getBeanNamesForType(ServerProperties.class);
-			Assert.state(serverPropertiesBeans.length == 1,
-					"Multiple ServerProperties beans registered " + StringUtils
-							.arrayToCommaDelimitedString(serverPropertiesBeans));
+			if (serverPropertiesBeans.length == 0) {
+				throw new IllegalStateException("No ServerProperties bean registered");
+			}
+			if (serverPropertiesBeans.length > 1) {
+				throw new IllegalStateException(
+						"Multiple ServerProperties beans registered " + StringUtils
+								.arrayToCommaDelimitedString(serverPropertiesBeans));
+			}
 		}
 
 	}