QuickJS: fix promise and event leak on proxy URL evaluation failure

QuickJS: fix promise and event leak on proxy URL evaluation failure

Previously, when dynamic proxy URL evaluation failed in
ngx_qjs_fetch(), the error was returned directly, bypassing the fail
path which deletes the registered fetch event and releases the
promise.  The njs counterpart already goes through the fail path.

As a side effect, the error is now reported by rejecting the
returned promise instead of throwing synchronously, consistently
with the other failure paths.