Fix error message

Concatenating the strings does not insert a space between them. Thus, instead of "a write of", we would get "awrite of", which ends up in the UI. A more comprehensive solution would be to only send a symbolic representation of the error, and let the frontend take care of creating a (customizable, translatable, ...) message from it. That is left for another day, though.

requested review from @fkosmale

assigned to @fkosmale

requested review from @fefrei and removed review request for @fkosmale

That's obviously a safe change. The question is whether everything will explode when we try to deploy it. We'll see

mentioned in commit 8d4f9da9

merged

It did not explode. Thanks