fix(scully): make sure scully emits non-zero exit code on errors

[beeman]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

Currently scully always emits a 0 as exit code, even when it fails. Emitting a non-zero exit code is essential if you want to use scully in an automated process, as it will halt the process if something fails - currently it just continues.

What is the new behavior?

Emit exit code 5 (Fatal exception) when something fails.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[SanderElias]

@beeman Thanks for the PR.
This one was already on my radar, thanks for handling it.
However, error number 5 is in my view not suitable. the node documentation says: There was a fatal unrecoverable error in V8 which is clearly not the case.

Can you change to 15? As this is not being used by any node internals, it sends out a clear signal that it is something we do exit on deliberate. We might even refactor to a couple of different codes in te futue

[beeman]

@SanderElias just updated the PR to emit 15

[aaronfrost]

@beeman what does a non-zero exit code do? I want to learn. Please teach me.

[beeman]

@aaronfrost this is relevant when we want to automate stuff, emitting a non-zero exit code from a child process will instruct the parent process to halt as something failed.

This is useful when you want to automate things in for instance a CI setup. There we want to execute all the steps, but we only want to continue with the next step if the current one successfully exits.

This is an example of npm emitting exit codes 0 or 1 (Generally running echo $? in a shell will show the exit code of the previous command):

➜  ~ npm install -g --silent @angular/cli
/usr/local/bin/ng -> /usr/local/lib/node_modules/@angular/cli/bin/ng
+ @angular/cli@8.3.21
updated 2 packages in 13.692s
➜  ~ echo $?
0
➜  ~ npm install -g --silent @angular/cli-this-does-not-work
➜  ~ echo $?
1