fix: merge-pr.sh emits output when bailing out

[coriolinus]

Turns out that shellcheck was right, and my bash-fu was weak. This now properly emits output to stderr instead of a file named 2.

Also it now uses printf instead of echo so that things like \n work.

What's new in this PR

---

PR Submission Checklist for internal contributors

• The PR Title
  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[github-actions]

Bencher Report

Branch	prgn/fix/output-on-merge-script-fail
Testbed	ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	milliseconds (ms)
Commit add f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	19.08
Commit add f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	6.97
Commit add f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	9.70
Commit add f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	12.04
Commit add f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	15.28
Commit add f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	24.07
Commit add f(number clients)/cs1/mem/1002	📈 view plot 🚷 view threshold	985.74
Commit add f(number clients)/cs1/mem/2	📈 view plot 🚷 view threshold	7.05
Commit add f(number clients)/cs1/mem/202	📈 view plot 🚷 view threshold	84.84
Commit add f(number clients)/cs1/mem/402	📈 view plot 🚷 view threshold	220.62
Commit add f(number clients)/cs1/mem/602	📈 view plot 🚷 view threshold	428.85
Commit add f(number clients)/cs1/mem/802	📈 view plot 🚷 view threshold	678.55
Commit pending proposals f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	116.56
Commit pending proposals f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	28.76
Commit pending proposals f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	46.43
Commit pending proposals f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	61.12
Commit pending proposals f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	79.97
Commit pending proposals f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	95.12
Commit pending proposals f(pending size)/cs1/mem/1	📈 view plot 🚷 view threshold	18.86
Commit pending proposals f(pending size)/cs1/mem/101	📈 view plot 🚷 view threshold	121.89
Commit pending proposals f(pending size)/cs1/mem/21	📈 view plot 🚷 view threshold	36.61
Commit pending proposals f(pending size)/cs1/mem/41	📈 view plot 🚷 view threshold	56.92
Commit pending proposals f(pending size)/cs1/mem/61	📈 view plot 🚷 view threshold	75.80
Commit pending proposals f(pending size)/cs1/mem/81	📈 view plot 🚷 view threshold	96.59
Commit remove f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	28.33
Commit remove f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	6.92
Commit remove f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	9.10
Commit remove f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	12.43
Commit remove f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	17.55
Commit remove f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	22.18
Commit remove f(number clients)/cs1/mem/1002	📈 view plot 🚷 view threshold	31.26
Commit remove f(number clients)/cs1/mem/2	📈 view plot 🚷 view threshold	138.23
Commit remove f(number clients)/cs1/mem/202	📈 view plot 🚷 view threshold	116.12
Commit remove f(number clients)/cs1/mem/402	📈 view plot 🚷 view threshold	94.33
Commit remove f(number clients)/cs1/mem/602	📈 view plot 🚷 view threshold	72.98
Commit remove f(number clients)/cs1/mem/802	📈 view plot 🚷 view threshold	51.80
Commit update f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	137.81
Commit update f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	7.07
Commit update f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	33.93
Commit update f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	60.05
Commit update f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	86.78
Commit update f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	112.82

🐰 View full continuous benchmarking report in Bencher