Improve checks title and content

[uhafner]

Currently, there are too many assumptions about the existing values. We need to add some guards to improve the title so that only meaningful results are actually produces.

• When there are modified lines, print coverages of those lines. Otherwise, fall back to project coverage.

Will close #580.

[codecov]

Codecov Report

Merging #587 (5078829) into master (d14733d) will increase coverage by 0.15%.
The diff coverage is 93.61%.

@@ Coverage Diff @@ ## master #587 +/- ## ============================================ + Coverage 72.16% 72.32% +0.15%  - Complexity 1550 1575 +25  ============================================ Files 127 127 Lines 6068 6059 -9 Branches 633 637 +4 ============================================ + Hits 4379 4382 +3  + Misses 1473 1456 -17  - Partials 216 221 +5 

Impacted Files	Coverage Δ
...gins/coverage/metrics/steps/CoverageViewModel.java	51.58% <ø> (ø)
...overage/metrics/steps/CoverageChecksPublisher.java	90.50% <92.85%> (-1.73%)	⬇️
...ugins/coverage/metrics/model/ElementFormatter.java	56.20% <100.00%> (+4.35%)	⬆️
...ns/coverage/metrics/steps/CoverageBuildAction.java	76.36% <100.00%> (+9.07%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more