More informative Slack notifications about pipelines

Currently:

virtkick/virtkick-webapp: Commit 24be3b12 of master branch by Nowaker failed in 981 seconds

Requested:

virtkick/virtkick-webapp: Commit 24be3b12 of master branch by Nowaker failed on stage deploy in 981 seconds

That is:

• Add failed stage name with a direct link to the build
• Add direct link to user.

Somewhat related to #21114 (moved). These guys are requesting a new feature. All I'm requesting is to pimp up the wording and links.