build_oai: exit early on error

There are a lot of case where the build scripts fails. Despite this, the
build script happily continue to the end and tells "Successfully built
xxxx".
This fix ensure the build stops early with the first error message on the
bottom of the log.

cmake_targets/build_oai

@@ -33,6 +33,7 @@
 # brief OAI automated build tool that can be used to install, compile, run OAI.
 # author  Navid Nikaein, Lionel GAUTHIER, Laurent Thomas
 
+set -e
 
 ################################
 # include helper functions