install_libspotify: Error out if any step fails

[chelmertz]

install_libspotify.sh is run by travis.ci in the before_script section. According to travis' documentation[1]:

If before_install, install or before_script return a non-zero
exit code, the build is errored and stops immediately.

Inexpensive safeguards are always nice to have, especially when they support the 'crash early' mantra.

1: https://docs.travis-ci.com/user/customizing-the-build/#Breaking-the-Build

Signed-off-by: Carl Helmertz helmertz@gmail.com

[catharsis]

Thank you!