Reduce likelihood of spurious merge conflict bot comment

Previously, the "Manage PRs" workflow made a comment suggesting to resolve the merge conflict after any failure to merge
a pull request. This comment is worded in a way that makes it somewhat applicable to other causes, but still might cause
the submitter to waste time unnecessarily trying to figure out how to merge a nonexistent merge conflict when the failure
had a different cause.

The 405 response is not specific to a failure due to merge conflict, but I believe that all failures due to merge
conflict will result in a 405. This means that the check is not perfect, but will make spurious mentions of merge
conflict resolution less likely at least.

A review is requested from a maintainer any time the merge fails, so they will be able to investigate and provide
assistance if necessary.

.github/workflows/manage-prs.yml

@@ -408,6 +408,7 @@ jobs:
     runs-on: ubuntu-latest
     outputs:
       pass: ${{ steps.merge.outcome == 'success' }}
+      status: ${{ steps.merge.outputs.status }}
 
     steps:
       - name: Approve pull request
@@ -482,7 +483,8 @@ jobs:
     if: needs.merge.outputs.pass == 'false'
     runs-on: ubuntu-latest
     steps:
-      - name: Comment on merge failure
+      - name: Comment on merge conflict
+        if: needs.merge.outputs.status == '405'
         uses: octokit/request-action@v2.x
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -500,6 +502,10 @@ jobs:
 
             Once that is done, it will be merged automatically.
 
+      - name: Fail on unidentified merge failure
+        if: needs.merge.outputs.status != '405'
+        run: exit 1 # Trigger the `unexpected-fail` job
+
   not-submission:
     needs:
       - parse