Server should return error code from client

[Javier Collado]

Right now when the client returns an error code, the server ignores it and
returns success (0).

This makes difficult from jenkins when a test run failed because of that. To
fix this, the server should return the error code from the client and the
client should return a failure when some test case in the test run failed.

In the future, a better integration with jenkins should provide detailed
results for every test case in the test run. However, that would be addressed
in a different bug. For now, just making sure that any failure is noticeable
in the jenkins results is good enough.