Don't use {} string notation

[David Planella]

Some of our translatable strings use Python's recommended {} notation for string substitution. It affects 3 strings:

- "A network connection error occured: {0}"
- "An error occured while trying to shorten the URL: {0}"
- "{1} {0}"

Unfortunately, gettext does not support this (*) notation and expects %s-style notation, so those strings will not be translated. We should fix that.

(*) To be precise, it supports this from yesterday onwards.