Improve the conformance to the draft OpenFormula specification.

T() now gives you back an empty cell if the argument is not a string, which means it works the same way that the specification requires (which the specification says is the way Excel and OOcalc work, and matches my testing on OOcalc and gnumeric).

The ROMAN() function now takes an optional conciseness argument, which prevents parse errors. We don't actually vary behaviour on this argument yet.

MID() now returns an error if the length argument is negative, as required by the specification - previously we just returned an empty string.