[docs] Scudo: document error messages & their potential cause

Summary:
A couple of changes in the Scudo documentation:
- tag the shell code blocks as `console`;
- document error messages that are displayed in some termination conditions,
  the reason they triggered, and potential causes.

Reviewers: eugenis, enh

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56857

llvm-svn: 351838