Cleanup, use ::error::, update README

[stertooy]

• Remove version number from README header (which is the case for all other gap-actions).
• Update action versions in README.
• Use ::error:: in echo statements right before exit 1, to make them more visible in GitHub Action logs.
• Replace GAP Error(" [...] ") by Exec("echo \"::error:: [...] \""); FORCE_QUIT_GAP(1); for the same reason.
• Made indendation consistent (2 spaces per level).
• Error if non-empty input testfile does not exist, instead of using TestFile from PackageInfo.g as a fallback (see The action should fail if the test file doesn't exist #8).