KIO HTTP: Store error string for deferred result
Otherwise we will emit an error without the corresponding text.
This fixes the favicontest.
Fixes
FAIL! : FavIconTest::failedDownloadShouldBeRemembered() Compared values are not the same
Actual (job->errorString()) : "The file or folder does not exist."
Expected (QStringLiteral("The file or folder https://kde.org/doesnotexist/favicon.ico does not exist.")): "The file or folder https://kde.org/doesnotexist/favicon.ico does not exist."
Loc: [/builds/frameworks/kio/autotests/favicontest.cpp(204)]