Fetch: fixed QuickJS conversion error handling

Fetch: fixed QuickJS conversion error handling

Preserve exceptions raised by QuickJS conversions and synthesize an out of
memory exception only for silent pool allocation failures in ngx_qjs_string()
callers.

Use JS_GetOpaque() where a class mismatch is a normal miss, avoiding a stale
pending TypeError while continuing with non-Headers and non-Request input.
Also release the headers init value before throwing for a non-object Headers
option.