[PATCH 0/2] Unify the style of user messages

While working on the translations, I came accross these strings. There doesn't seem to be a rule on how these messages should be formatted (or there is, then I didn't look well enough).

To me it feels better to standardize the style, so I looked in the sourcecode for which style is used most and composed some guidelines to unify the messages:
    - messages start with a capital,
    - short messages do not end with a full stop,
    - paths, filenames, and commands are quoted by single quotes (if not separated by the normal text by a ':'),
    - 'could not' is used rather than 'cannot'. 

There could of course be much more changes, but I don't know whether these changes are appreciated. Also, the guidelines might be disputable. Maybe it is not the best timing because we are in rc phase and the people start to translate. On the other hand, we might better change it before all translations are finished.

The second patch makes a few string translatable. 

Vincent