Use response status 500 if search failed but wasn't syntax error

Closes #3026

(cherry picked from commit 652421c)

graylog2-server/src/main/java/org/graylog2/rest/resources/search/SearchResource.java

@@ -57,6 +57,8 @@
 import javax.inject.Inject;
 import javax.ws.rs.BadRequestException;
 import javax.ws.rs.ForbiddenException;
+import javax.ws.rs.InternalServerErrorException;
+import javax.ws.rs.WebApplicationException;
 import javax.ws.rs.core.Response;
 import java.io.IOException;
 import java.util.ArrayList;
@@ -246,7 +248,7 @@ protected ChunkedOutput<ScrollResult.ScrollChunk> buildChunkedOutput(final Scrol
         return output;
     }
 
-    protected BadRequestException createRequestExceptionForParseFailure(String query, SearchPhaseExecutionException e) {
+    protected WebApplicationException createRequestExceptionForParseFailure(String query, SearchPhaseExecutionException e) {
         LOG.warn("Unable to execute search: {}", e.getMessage());
 
         QueryParseError errorMessage = QueryParseError.create(query, "Unable to execute search", e.getClass().getCanonicalName());
@@ -297,12 +299,14 @@ protected BadRequestException createRequestExceptionForParseFailure(String query
                         parseException.getClass().getCanonicalName());
                 }
             }
-        }
 
         return new BadRequestException(Response
             .status(Response.Status.BAD_REQUEST)
             .entity(errorMessage)
             .build());
+        } else {
+            return new InternalServerErrorException("Unable to fulfill search request", e);
+        }
     }
 
     public void checkSearchPermission(String filter, String searchPermission) {