Raise exception on MediaWiki errors instead of ignoring them silently

[Peter Hozák]

We silently ignore HTTP errors (401, 500, ...) and MediaWiki errors in https://github.com/robertskmiles/stampy/blob/5663c7876b7df9a3ed0626ee6ec00b83a82e1ef3/api/semanticwiki.py#L56-L61

This may or may not be a root cause or recent problems that we cannot move pages in production, while it works on my local machine.

Let's raise an exception and it will be posted to Discord (already done by existing code for unhandled exception in this module).