Use parsed HTML error responses instead of api-error-*
There is no point in keeping these api-error- messages.
They were originally introduced in UploadWizard, then moved
to core, where nothing else (apart from some in BookletLayout)
uses them. They are mostly duplicates of other messages in
core, and these are very easy to miss when updating a message
Instead of building the messages on the frontend, we're much
better off requesting a parsed error response from the API
right away. This also lets us get rid of sime hacks (e.g. for
And then a few other api-error-* messages have been removed
in favor of their alternatives (which are also used elsewhere)
And stashfailed is not a warning. Maybe it was at some point,
but stashfailed now dies with an error.