stbt batch report: Ignore invalid stbt-run.json files

If *any* of the test-run directories contained an invalid
`stbt-run.json` file, `stbt batch report` would fail to generate the
top-level html report because of "ValueError: No JSON object could be
decoded".
stbt batch report: Cleanup internal variable name

This function (`read_run_dir.load_json`) is used for more than just
"stbt-run.json", so the previous variable name was misleading.