Return 400 when handling invalid JSON (#49558)

Backport of #49552.

Closes #49428. The code that works out an HTTP code for an exception didn't
consider the JsonParseException case, meant that an invalid JSON request could
result in a 500 Internal Server Error. Now it returns 400 Bad Request.

server/src/main/java/org/elasticsearch/ExceptionsHelper.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch;
 
+import com.fasterxml.jackson.core.JsonParseException;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.apache.lucene.index.CorruptIndexException;
@@ -71,6 +72,8 @@ public static RestStatus status(Throwable t) {
                 return ((ElasticsearchException) t).status();
             } else if (t instanceof IllegalArgumentException) {
                 return RestStatus.BAD_REQUEST;
+            } else if (t instanceof JsonParseException) {
+                return RestStatus.BAD_REQUEST;
             } else if (t instanceof EsRejectedExecutionException) {
                 return RestStatus.TOO_MANY_REQUESTS;
             }

server/src/test/java/org/elasticsearch/ExceptionsHelperTests.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch;
 
+import com.fasterxml.jackson.core.JsonParseException;
 import org.apache.commons.codec.DecoderException;
 import org.apache.lucene.index.CorruptIndexException;
 import org.elasticsearch.action.OriginalIndices;
@@ -95,6 +96,7 @@ private void assertError(final Throwable cause, final Error error) {
 
     public void testStatus() {
         assertThat(ExceptionsHelper.status(new IllegalArgumentException("illegal")), equalTo(RestStatus.BAD_REQUEST));
+        assertThat(ExceptionsHelper.status(new JsonParseException(null, "illegal")), equalTo(RestStatus.BAD_REQUEST));
         assertThat(ExceptionsHelper.status(new EsRejectedExecutionException("rejected")), equalTo(RestStatus.TOO_MANY_REQUESTS));
     }