[Github][CI] Make the new premerge system fail jobs on errors

This patch makes the new premerge system report failures when the build
errors out. We were previously not doing this to not notify people on
failures as we were testing out the infra.

This works towards making the new premerge system canonical and the
deprecation of the old system.

A launch announcement on Discourse will accompany this commit.

Pull Request: llvm#139359

Author: boomanaiden154

.github/workflows/premerge.yaml

@@ -42,9 +42,6 @@ jobs:
         # Mark the job as a success even if the step fails so that people do
         # not get notified while the new premerge pipeline is in an
         # experimental state.
-        # TODO(boomanaiden154): Remove this once the pipeline is stable and we
-        # are ready for people to start recieving notifications.
-        continue-on-error: true
         run: |
           git config --global --add safe.directory '*'
 
@@ -109,9 +106,6 @@ jobs:
         # Mark the job as a success even if the step fails so that people do
         # not get notified while the new premerge pipeline is in an
         # experimental state.
-        # TODO(boomanaiden154): Remove this once the pipeline is stable and we
-        # are ready for people to start recieving notifications.
-        continue-on-error: true
         if: ${{ steps.vars.outputs.windows-projects != '' }}
         shell: cmd
         run: |