Display line number in warnings about broken cross-references

[mfisher87]

Proposal

I'm getting some warnings about cross-references in my ~500-line document:

⚠️   04-materials/02-anatomy-of-extensions.md Cross reference target was not found: term-plugins <plugin
⚠️   04-materials/02-anatomy-of-extensions.md Cross reference text is empty for <undefined>

I have no idea where to find the offending cross references! Line numbers would go a long way, especially for that 2nd warning :)