Fix duplicated error messages for multi init locs in mCRL2 transformation

The mCRL2 transformation pre-checker reports multiple initial locations for every n-1 possible initial locations.

automaton Test:
    location One:
        initial;
    location Two:
        initial;
    location Three:
        initial;
    location Four:
        initial;
end

ERROR: CIF to mCRL2 transformation failed due to unsatisfied preconditions:
 - Automaton "Test" has more than one initial location.
 - Automaton "Test" has more than one initial location.
 - Automaton "Test" has more than one initial location.