Enhancement about relative assets in markdown

[Mister-Hope]

Currently in markdown when importing an non-exist image in markdown, VuePress will throw an error and stop building and previewing. I know this is expected because these files are treated as module and pass through loaders.

![alt](./not-exist.png)

But I am wondering if we provide fallback for images could be better. When we are requiring a image extension file in image tag (Staring with @source by markdown-it parser), we can just provide a fallback image (E.g.: A default image showing "Image don't exist") or just stop parsing it. Then we can output an error showing this link is not existed.

After this, users will get error in console or build output log telling their image ./not-exist.png can not found in xxx file and still get a dev server preview or build result.

One strong reason is that a broken image is not critical enough to stop the whole build or dev process.

[Mister-Hope]

@meteorlxy Maybe check this one ?