provide help & links for some build errors

[syphar]

coming from things like this issue.

There are certain errors where we as maintainers (or more experienced crate maintainers) directly see what's the issue.

What we could do is to search for certain patterns (just with a regex), and provide an explanation of the likely problem.

Examples (there's probably more):

• kind: ReadOnlyFilesystem -> link to https://docs.rs/about/builds#read-only-directories
• sigkill -> memory issues
• timeouts?

If we include our thoughts in #878, we could also make this even better by storing the specific rustwide error (if it's enough?) and using that.