Make use of exit codes to check if things are behaving as expected

[Kurolox]

Right now the bot depends only on the stdout and stderr to know if things went well (if there's any output while compiling, the bot will assume that there was a problem and it will return the compiler output without executing the binary, even if it was just a warning.)

Adding support for checking the exit code of the subprocess would improve the bot and make it more flexible.