Ungraceful exit when fit parameter not constrained

[Michael Keith]

Situation: Unconstrained fit parameter, e.g. a jump on a non-existant flag.

Expected Result: tempo2 should warn the user that the fit parameter is not constrained and either disable the fit parameter or refuse to do the fit.

Actual Result: assert failure and a non-helpful error message.

[Michael Keith]

Fixed - now prints warnings and cleanly aborts the fit.