Issue #80: Raise build error on failed shell command.

[guncha25]

Make build more predictable by raising error if shell exited with non zero exit status.
Fixes list shell commands.

[ragnarkurmwunder]

I see the PR is good refactor.
Does it output stdout and stderr of failed command?
It does not report error code, which is valuable in some cases.

[guncha25]

It outputs failed command. I think error is outputted by itself. Its was just not clear on what command error occurred.

[ragnarkurmwunder]

For the error code part, I would suggest something like this:

exit_code = os.system(command)
if exit_code <> 0:
    raise BuildError("Failed executing: '" + command + "' (Exit code: " + str(exit_code) + ")")

Note that I have included additional single quotes around the command for better visual.