Provide informative reason when rejecting request with invalid level

Previously, bad request with no reason was included in the response.
This commit introduces the reason when invalid log level is sent in the
request.

Fixes gh-10588

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/LoggersMvcEndpoint.java

@@ -22,10 +22,12 @@ import org.springframework.boot.actuate.endpoint.LoggersEndpoint;
 import org.springframework.boot.actuate.endpoint.LoggersEndpoint.LoggerLevels;
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.boot.logging.LogLevel;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.ResponseBody;
+import org.springframework.web.bind.annotation.ResponseStatus;
 
 /**
  * Adapter to expose {@link LoggersEndpoint} as an {@link MvcEndpoint}.
@@ -74,7 +76,7 @@ public class LoggersMvcEndpoint extends EndpointMvcAdapter {
 			return ResponseEntity.ok().build();
 		}
 		catch (IllegalArgumentException ex) {
-			return ResponseEntity.badRequest().build();
+			throw new InvalidLogLevelException("No such log level " + configuration.get("configuredLevel"));
 		}
 	}
 
@@ -83,4 +85,17 @@ public class LoggersMvcEndpoint extends EndpointMvcAdapter {
 		return (level == null ? null : LogLevel.valueOf(level.toUpperCase()));
 	}
 
+	/**
+	 * Exception thrown when the specified log level cannot be found.
+	 */
+	@SuppressWarnings("serial")
+	@ResponseStatus(value = HttpStatus.BAD_REQUEST, reason = "No such log level")
+	public static class InvalidLogLevelException extends RuntimeException {
+
+		public InvalidLogLevelException(String string) {
+			super(string);
+		}
+
+	}
+
 }

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/mvc/LoggersMvcEndpointTests.java

@@ -49,6 +49,7 @@ import org.springframework.test.web.servlet.setup.MockMvcBuilders;
 import org.springframework.web.context.WebApplicationContext;
 
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
@@ -174,7 +175,8 @@ public class LoggersMvcEndpointTests {
 	public void setLoggerWithWrongLogLevel() throws Exception {
 		this.mvc.perform(post("/loggers/ROOT").contentType(MediaType.APPLICATION_JSON)
 				.content("{\"configuredLevel\":\"other\"}"))
-				.andExpect(status().is4xxClientError());
+				.andExpect(status().is4xxClientError())
+				.andExpect(status().reason(is("No such log level")));
 		verifyZeroInteractions(this.loggingSystem);
 	}