[travis] detect errors in docker test

Summary:
As noticed by technion in https://github.com/facebook/infer/issues/570,
failures in the Docker test do not result in failures of the test. Lol.

Reviewed By: mbouaziz

Differential Revision: D4605364

fbshipit-source-id: de9f84c

.travis/docker_tests.sh

@@ -7,9 +7,12 @@
 # LICENSE file in the root directory of this source tree. An additional grant
 # of patent rights can be found in the PATENTS file in the same directory.
 
+set -e
 
 SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 
+set -x
+
 # assumes we are in .travis/
 cd "${SCRIPT_DIR}/../docker"