Add error causes to diagnostic message

[remcohaszing]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

Currently error causes are omitted. This makes it hard to troubleshoot certain problems. (See remarkjs/vscode-remark#135)

Current solutions

Run remark from the command line to see the cause.

Proposed solutions

Append the error cause to the diagnostic message.