Rely on last command exit status to raise error

[gquemener]

Some tools like composer write human-related non-error messages to
stderr, thus it's not safe to rely on stderr to decide if the command
failed.

More info:

• All messages are sent to stderr composer/composer#3795 (comment)
• Error output printed to STDOUT instead of STDERR composer/composer#1905 (comment)
• stderr output using writeError(), closes #1905 composer/composer#3715 (comment)

Inspiration:

• http://stackoverflow.com/questions/10478491/php-ssh2-exec-channel-exit-status
• http://stackoverflow.com/questions/18278023/php-net-ssh2-exec-how-to-get-the-command-return-code

[gquemener]

If you'd have some time to review it @Herzult, that'd be great, please :)

[Herzult]

You are right, exit code is more appropriate to determine wether a command was successful or not, thanks ! :)