fix(md2html): check curl exit status for network errors

[ekalinin]

Summary

For a local file or stdin, a network failure was not detected: gh-md-toc printed an
empty TOC and exited with code 0. With --insert the empty TOC replaced the existing
one in the file.

gh_toc_md2html checked $? after rm -f "${gh_file_md}~~", which 8daa73c ("Skip
headers if end marker is present") inserted between the curl call and the check. So
$? was the status of rm -f, always 0, and XXNetworkErrorXX was never returned.

• gh-md-toc: save the curl exit status right after the call and check it instead
  of $?.
• tests/tests.bats: new test Error for local file without network access. It sends
  curl through HTTPS_PROXY=http://127.0.0.1:9 (a closed port) and expects the
  "requires access to github API" message and a non-zero exit code.

Part of #167: this resolves the SC2181 warning.

Testing

• Without the fix the new test fails (empty TOC, exit code 0); with the fix all 15
  tests pass.
• shellcheck -e SC2008 gh-md-toc no longer reports SC2181; the other warnings from
  make lint fails on shellcheck style and info warnings #167 are unchanged.