Adds error message with filename on Markdown error, fixes bug in panicOnBuild

[mslooten]

As discussed in #8524, when Markdown isn't correctly processed an error is thrown which doesn't include the filename. This fixes it using a try/catch with the reporter.panicOnBuild function, which is also changed since it had unexpected behavior (it didn't terminate the build, but it did terminate on other commands such as gatsby develop).

---edit by @pieh:
closes #8524

[pieh]

I forgot about something earlier - markdown doesn't always come from file, so we will need to check if node.absolutePath is set before using it. For example:

`Error processing Markdown ${node.absolutePath ? `file: ${node.absolutePath}` : ``}:\n`

[pieh]

Looking good, only one last small thing to fix (my bad for not thinking about it earlier when I left some comments in the issue)

[mslooten]

@pieh Thanks for pointing that out – hadn't considered that. Changed it up a bit so it checks if the path is available.

[ahinrichs]

What about giving some fallback identifier like node.id? This could help to find the error if the markdown isn't a file.

[pieh]

What about giving some fallback identifier like node.id? This could help to find the error if the markdown isn't a file.

It definitely won't hurt. id will often be something like UUID (so not human readable), but I guess it's better feedback that none.

[mslooten]

Cool, can show id as fallback. Can I assume there's always an id available or should that also check for that presence?

[ahinrichs]

BTW, you could also on the fly start using the new createContentDigest API. See #8805.

[pieh]

BTW, you could also on the fly start using the new createContentDigest API. See #8805.

Let's not derail this PR into adding more stuff here :)

Cool, can show id as fallback. Can I assume there's always an id available or should that also check for that presence?

id is required, so it will always be there - it's safe to use that without any checks

[mslooten]

@pieh Great, thanks! Added the change to show the node.id.

[pieh]

Thanks @mslooten!

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