change loader error handling

[e271828-]

Summary

• Normalize SDK script-load failures to script-error.
• Report script-load-error-other if the initial onError handler throws.
• Prevent loader rejection values from ever leaking through onError.

Motivation

• Existing promise chaining routed exceptions thrown by initial error handling back through handleError.
• hcaptcha-loader has more error states, but the relevant error is that the script couldn't be loaded.

Testing

Used 'manual' Chromium test with unreachable scriptSource to confirm:

• Normal handler: script-error
• Throwing handler: script-error, then script-load-error-other
• Initial request plus two retries failed as expected.