Diagnostics for `hashFiles` failing on a broken symlink don't describe the problem

Describe the bug

Currently if hashFiles hits a broken symlink it fails the step hard with an opaque error: it says that there is a problem, but no guidance about what it was. For instance:

Error: The template is not valid. .github/workflows/test.yml (Line: 14, Col: 14): hashFiles('*') failed. Fail to hash files under directory '/home/runner/work/GitHub-runner-broken-symlink-diagnostics/GitHub-runner-broken-symlink-diagnostics'

That is all it says, without debug logging. This is confusing and frustrating, and is even worse with a glob like some-directory/**/*: who knows which file somewhere under that directory is the problem.

Given the job is failing, preferably this would do at least one of:

1. include which file(s) is causing the problem
2. suggest how to find more information, e.g. "turn on debug logging".

Debug logging helps with this, somewhat, if one knows what to look for (I had to find https://github.com/actions/runner/issues/1666 to realise I could search for ENOENT in the logs, since there was too much output to look at manually):

##[debug]Error: ENOENT: no such file or directory, stat '/home/runner/work/GitHub-runner-broken-symlink-diagnostics/GitHub-runner-broken-symlink-diagnostics/broken'
##[debug]    at Object.statSync (node:fs:1583:3)
##[debug]    at /home/runner/runners/2.311.0/bin/hashFiles/index.js:84:24
##[debug]    at Generator.next (<anonymous>)
##[debug]    at fulfilled (/home/runner/runners/2.311.0/bin/hashFiles/index.js:35:58) {
##[debug]  errno: -2,
##[debug]  syscall: 'stat',
##[debug]  code: 'ENOENT',
##[debug]  path: '/home/runner/work/GitHub-runner-broken-symlink-diagnostics/GitHub-runner-broken-symlink-diagnostics/broken'
##[debug]}

To Reproduce

1. Have a broken symlink somehow
2. Have a hashFiles call in the workflow that includes that symlink
3. See error and then struggle to understand what needs to be changed to fix the problem

Example: https://github.com/huonw/GitHub-runner-broken-symlink-diagnostics

• run without debug logging: https://github.com/huonw/GitHub-runner-broken-symlink-diagnostics/actions/runs/7028745161/job/19125215085
• run with debug logging: https://github.com/huonw/GitHub-runner-broken-symlink-diagnostics/actions/runs/7028745161/job/19125266914

Expected behavior

It would be better if the error message said:

1. which files are a problem (preferably if there's multiple files that are a problem, highlight them all)
2. why they're a problem (e.g. "symlink to file that doesn't exist")

1 is most critical, but 2 would be great.

Runner Version and Platform

Current runner version: '2.311.0'
Operating System
  Ubuntu
  22.04.3
  LTS

What's not working?

The error is opaque, currently:

Error: The template is not valid. .github/workflows/test.yml (Line: 14, Col: 14): hashFiles('*') failed. Fail to hash files under directory '/home/runner/work/GitHub-runner-broken-symlink-diagnostics/GitHub-runner-broken-symlink-diagnostics'

Job Log Output

• run without debug logging: https://github.com/huonw/GitHub-runner-broken-symlink-diagnostics/actions/runs/7028745161/job/19125215085
• run with debug logging: https://github.com/huonw/GitHub-runner-broken-symlink-diagnostics/actions/runs/7028745161/job/19125266914

Runner and Worker's Diagnostic Logs

N/A

Start by reproducing the broken-symlink workflow described in the issue, comparing the normal and debug logs. Trace the hashFiles entry point and its error-reporting path; done means the failure identifies the problematic file(s) and, where feasible, explains the cause or directs users to useful diagnostics.