Exceptions overhaul

[xeioex]

Cleaning up of exception classes for QuickJS/njs integration code + small fixes found. The goal is not to rewrite
all messages, but to make exception classes predictable while keeping real bug fixes visible for review and backports.

• Wrong receiver and wrong user-visible API usage are TypeError.
• Numeric/status bounds violations are RangeError.
• Host/runtime failures remain InternalError.
• Allocation-only failures use memory errors / OutOfMemory.
• Correctness fixes are split from pure exception-class retyping when the split is small and clear.

Correctness fixes split out

• Fetch QuickJS conversion handling:
  • preserve exceptions already raised by QuickJS conversions;
  • add JS_HasException() / JS_ThrowOutOfMemory() guards where ngx_qjs_string() can fail silently on pool allocation;
  • use JS_GetOpaque() instead of JS_GetOpaque2() where class mismatch is a normal miss, avoiding stale pending exceptions;
  • free the headers init value before throwing for non-object Headers.
• HTTP njs handlers:
  • set pending exceptions before returning NJS_ERROR from response output paths such as sendHeader(), send(), and finish().
• Stream variables:
  • clear vv->valid and set memory error when stream variable storage allocation fails.

Exception-class changes

• Fetch, HTTP, Stream, XML, and common helper code now use TypeError for receiver/API misuse.
• Fetch/HTTP/Stream status and code range checks use RangeError.
• XML parse failures use SyntaxError.
• Internal nginx/output/body collection failures remain InternalError.
• ngx_js_ext_log() / ngx_qjs_ext_log() missing external receiver/context is treated as TypeError, matching the wrong-receiver policy.
• Stream s.done() while filtering is treated as TypeError, matching other wrong-phase user API calls.

Deliberate non-goals

• Shared dictionary exception alignment is not included as it requires additional work
• Existing generic conversion messages in HTTP/Stream handlers are mostly kept to avoid unnecessary user-visible churn.
• No extra minor receiver tests were added just to exercise every retyped branch.

[VadimZhestikov]

Two suggestions, otherwise looks good:

1. QJS/njs asymmetry (commit 3): ngx_qjs_request_ctor uses JS_ThrowInternalError for "input is not string or a Request
   object" where the njs path uses njs_vm_type_error. Should be JS_ThrowTypeError.
2. Minor classification inconsistency (commit 5): "failed to convert uri arg" / "failed to convert options.method"
   etc. are labeled TypeError but the conversion failure at those points is almost always OOM (the value type was
   verified earlier). Removing these explicit messages (as done elsewhere in the PR) would be cleaner and let njs set the
   right error type.

[xeioex]

QJS/njs asymmetry (commit 3): ngx_qjs_request_ctor uses JS_ThrowInternalError for "input is not string or a Request object" where the njs path uses njs_vm_type_error. Should be JS_ThrowTypeError.

Is fixed by 16f8534.