Add more detailed upload stash error messages

There are seven (used) error types in the stash class, and we umbrella'd
them all into one error message, which is mighty silly. This should give
us more information.

Also added to the mw.Api.errors list so UploadWizard can handle them.

Change-Id: I79bf0c29a4cef19363d111cc1128e35256ae572a
Merge "Add more detailed upload stash error messages"