web-ext submit: Source code submission failure should not result in the signed xpi getting lost

[Rob--W]

(Extracted out of #2868 (review) )

web-ext sign is expected to return the xpi (or instructions on downloading) when available.
In mozilla/addons-server#2859 (where the source code uploading feature was added), it's possible to fail web-ext sign when the source code fails to upload. That shouldn't happen; when there is a signed xpi available, the xpi should be returned (or at least the instructions to retrieve it).

Side note: developers should already be prepared to deal with a not-immediately-available xpi file; when signing is delayed for so long that web-ext gives up and recommends the user to wait for an e-mail.

[valenvb-ag]

It's maybe worth mentioning as well that the addons server will often return an error (504), even when the source was successfully uploaded. (mozilla/addons#1846), which makes this behaviour even more problematic.