Cleanup, ::error:: usage, README updates

[stertooy]

• Update README with recommendations regarding codecov-action (see Recommend using codecov/codecov-action with the input fail_ci_if_error: true #24).
• Update action versions in README.
• Replace GAP Error by Exec("echo \"::error:: [...] \""); FORCE_QUIT_GAP(1); to make the errors more visible in GitHub Action logs.
• Made indendation consistent (2 spaces per level).