Match git fsck error lines by prefix in --check

Substring-matching "error" against the full fsck output could false-positive on benign dangling/warning lines that happened to contain that word (e.g. a branch or path with "error" in it). Scan line-by-line for "error:" / "fatal:" / "missing " / "broken link" / "bad " prefixes, which is what git itself uses to signal real problems.