[SPARK-24660][SHS] Show correct error pages when downloading logs

## What changes were proposed in this pull request?

SHS is showing bad errors when trying to download logs is not successful. This may happen because the requested application doesn't exist or the user doesn't have permissions for it, for instance.

The PR fixes the response when errors occur, so that they are displayed properly.

## How was this patch tested?

manual tests

**Before the patch:**
 1. Unauthorized user
![screen shot 2018-06-26 at 3 53 33 pm](https://user-images.githubusercontent.com/8821783/41918118-f8b37e70-795b-11e8-91e8-d0250239f09d.png)

 2. Non-existing application
![screen shot 2018-06-26 at 3 25 19 pm](https://user-images.githubusercontent.com/8821783/41918082-e3034c72-795b-11e8-970e-cee4a1eae77f.png)

**After the patch**
 1. Unauthorized user
![screen shot 2018-06-26 at 3 41 29 pm](https://user-images.githubusercontent.com/8821783/41918155-0d950476-795c-11e8-8d26-7b7ce73e6fe1.png)

 2. Non-existing application
![screen shot 2018-06-26 at 3 40 37 pm](https://user-images.githubusercontent.com/8821783/41918175-1a14bb88-795c-11e8-91ab-eadf29190a02.png)

Author: Marco Gaido <marcogaido91@gmail.com>

Closes #21644 from mgaido91/SPARK-24660.

core/src/main/scala/org/apache/spark/status/api/v1/ApiRootResource.scala

@@ -28,7 +28,7 @@ import org.glassfish.jersey.server.ServerProperties
 import org.glassfish.jersey.servlet.ServletContainer
 
 import org.apache.spark.SecurityManager
-import org.apache.spark.ui.SparkUI
+import org.apache.spark.ui.{SparkUI, UIUtils}
 
 /**
  * Main entry point for serving spark application metrics as json, using JAX-RS.
@@ -148,38 +148,18 @@ private[v1] trait BaseAppResource extends ApiRequestContext {
 }
 
 private[v1] class ForbiddenException(msg: String) extends WebApplicationException(
-  Response.status(Response.Status.FORBIDDEN).entity(msg).build())
+    UIUtils.buildErrorResponse(Response.Status.FORBIDDEN, msg))
 
 private[v1] class NotFoundException(msg: String) extends WebApplicationException(
-  new NoSuchElementException(msg),
-    Response
-      .status(Response.Status.NOT_FOUND)
-      .entity(ErrorWrapper(msg))
-      .build()
-)
+    UIUtils.buildErrorResponse(Response.Status.NOT_FOUND, msg))
 
 private[v1] class ServiceUnavailable(msg: String) extends WebApplicationException(
-  new ServiceUnavailableException(msg),
-  Response
-    .status(Response.Status.SERVICE_UNAVAILABLE)
-    .entity(ErrorWrapper(msg))
-    .build()
-)
+    UIUtils.buildErrorResponse(Response.Status.SERVICE_UNAVAILABLE, msg))
 
 private[v1] class BadParameterException(msg: String) extends WebApplicationException(
-  new IllegalArgumentException(msg),
-  Response
-    .status(Response.Status.BAD_REQUEST)
-    .entity(ErrorWrapper(msg))
-    .build()
-) {
+    UIUtils.buildErrorResponse(Response.Status.BAD_REQUEST, msg)) {
   def this(param: String, exp: String, actual: String) = {
     this(raw"""Bad value for parameter "$param".  Expected a $exp, got "$actual"""")
   }
 }
 
-/**
- * Signal to JacksonMessageWriter to not convert the message into json (which would result in an
- * extra set of quotes).
- */
-private[v1] case class ErrorWrapper(s: String)

core/src/main/scala/org/apache/spark/status/api/v1/JacksonMessageWriter.scala

@@ -68,10 +68,7 @@ private[v1] class JacksonMessageWriter extends MessageBodyWriter[Object]{
       mediaType: MediaType,
       multivaluedMap: MultivaluedMap[String, AnyRef],
       outputStream: OutputStream): Unit = {
-    t match {
-      case ErrorWrapper(err) => outputStream.write(err.getBytes(StandardCharsets.UTF_8))
-      case _ => mapper.writeValue(outputStream, t)
-    }
+    mapper.writeValue(outputStream, t)
   }
 
   override def getSize(

core/src/main/scala/org/apache/spark/status/api/v1/OneApplicationResource.scala

@@ -140,11 +140,8 @@ private[v1] class AbstractApplicationResource extends BaseAppResource {
         .header("Content-Type", MediaType.APPLICATION_OCTET_STREAM)
         .build()
     } catch {
-      case NonFatal(e) =>
-        Response.serverError()
-          .entity(s"Event logs are not available for app: $appId.")
-          .status(Response.Status.SERVICE_UNAVAILABLE)
-          .build()
+      case NonFatal(_) =>
+        throw new ServiceUnavailable(s"Event logs are not available for app: $appId.")
     }
   }
 

core/src/main/scala/org/apache/spark/ui/UIUtils.scala

@@ -21,6 +21,7 @@ import java.net.URLDecoder
 import java.text.SimpleDateFormat
 import java.util.{Date, Locale, TimeZone}
 import javax.servlet.http.HttpServletRequest
+import javax.ws.rs.core.{MediaType, Response}
 
 import scala.util.control.NonFatal
 import scala.xml._
@@ -566,4 +567,8 @@ private[spark] object UIUtils extends Logging {
         NEWLINE_AND_SINGLE_QUOTE_REGEX.replaceAllIn(requestParameter, ""))
     }
   }
+
+  def buildErrorResponse(status: Response.Status, msg: String): Response = {
+    Response.status(status).entity(msg).`type`(MediaType.TEXT_PLAIN).build()
+  }
 }