[web] Return useful error messages when exceptions arise in ajax controllers (closes #3932503) The call to super() in RemoteCallException passes 'reason' all the way to python's Exception.__init__() which is then stored in Exception.args. This way, CubicWebPublisher.ajax_error_handler() gets a useful error message when calling unicode(exc). The change to uilib is just to prepend the exception type name. Just a small improvement.

[test] Fix test breakage uncovered by previous changeset Now that Binary.__eq__ doesn't always return true, this test started failing. CWAttribute.defaultval is zpickled, and the description_format attribute is a String, meaning unicode, so adjust the expected test result.

[etwist] use more specific REQUEST_ENTITY_TOO_LARGE instead of BAD_REQUEST When a request body exceeds the configured limit, return 413 to the client instead of a generic 400.

[entities, view] delete dead code The auto_unwrap_bw_compat metaclass calls unwrap_adapter_compat, which was removed in changeset 697a8181ba30 "remove 3.9 bw compat". So this can't possibly work anymore, meaning we can get rid of it.