#5908: Check for `tr_loadFile` return value instead of `errno` in `tr_variantFromFile`

#5908: Check for `tr_loadFile` return value instead of `errno` in `tr_variantFromFile`

Seems like there could be a defect in uClibc making errno not
thread-local. Don't rely on errno value but check function return value
instead which is a better failure indicator.

Return errors from `tr_loadFile` and `tr_variantFromFile` via tr_error.
Fix `tr_sessionLoadSettings` to not fail on Windows if settings.json
does not exist.