Fail consistently with non-existing packages

Make error reporting more consistent with non-existing packages.

Before:
 1. `pkg install non-existent`
    - Displays that the "non-existent" package does not exist;
    - returns 70 (EX_SOFTWARE).
 2. `pkg install apache22 non-existent`
    - Displays that the "non-existent" package does not exist (however
      it's harder to catch it);
    - Installs apache22;
    - returns 0 (success while the result is not what we expect).

This provide consistent failure (as in 1) when some packages are not
found.

While here, add a missing line break at the end of the error message for
inexistent packages.