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

Author: eddumelendez

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.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 Object set(@PathVariable String name,
 			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 @@ private LogLevel getLogLevel(Map<String, String> configuration) {
 		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.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 void setLoggerWhenDisabledShouldReturnNotFound() throws Exception {
 	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);
 	}