Fix handling of internal errors when uploading theming files

Fix handling of internal errors when uploading theming files

When a file can not be uploaded in the Theming app due to an expected
error (like an invalid mime type) the response contains a "data.message"
field. However, if the upload fails due to an unexpected error (like an
internal error, for example if the "mime_content_type" function is
disabled) there is no such field, so it should not be used.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>