workflows/labels: add more error context

The original error is kept, but the PR number is printed as well. Makes
debugging much easier.

.github/workflows/labels.yml

@@ -100,6 +100,7 @@ jobs:
                 ...prEventCondition
               },
               async (response, done) => (await Promise.allSettled(response.data.map(async (pull_request) => {
+                try {
                   const log = (k,v) => core.info(`PR #${pull_request.number} - ${k}: ${v}`)
 
                   log('Last updated at', pull_request.updated_at)
@@ -188,9 +189,12 @@ jobs:
                       labels: added
                     })
                   }
+                } catch (cause) {
+                  throw new Error(`Labeling PR #${pull_request.number} failed.`, { cause })
+                }
               })))
               .filter(({ status }) => status == 'rejected')
-              .map(({ reason }) => core.setFailed(reason))
+              .map(({ reason }) => core.setFailed(`${reason.message}\n${reason.cause.stack}`))
             )
 
       - uses: actions/labeler@8558fd74291d67161a8a78ce36a881fa63b766a9 # v5.0.0