Fix error properties

[ehmicky]

The exitCode and exitCodeName are currently wrong.

We use error.code as the base for exitCode and exitCodeName. We also use util.getSystemErrorName() on an exit code. Both are erroneous as they are mixing exit codes and error.code/error.errno, which are different things. error.code and error.errno are based on OS-specific system calls. Those are sometimes used as exit codes, but an exit code does not always translate to an error.errno.

Assigning a name or description to exit codes is tricky. That's because exit codes meaning is both application-specific and OS-specific. For example, the exit code 2 means "Command line usage error" if the command is bash. But it means "ENOENT - No such file or directory" if the error was due to a Linux system call error.

This PR fixes this by:

• removing error.exitCodeName (breaking change).
• not making exitCode use error.code anymore
• keeping error.code when defined instead of deleting it
• adding the error message Command failed with ${error.code} when error.code is defined
• improving error-related tests
• renaming the internal variable code to exitCode for clarity