SolveMain() silently reports success when -a auto-refinement never converges

[dhgaddy]

CSolveCap::SolveMain()'s auto-refinement (-a) loop only assigns
capRe/capIm on its convergence break:

if(diff <= globalVars.m_dAutoMaxErr) {
    capRe = cRe[i];
    capIm = cIm[i];
    break;
}

That's the only assignment in the loop. If it runs out all
SOLVE_MAX_AUTO_ITERATIONS rounds without ever hitting this break,
the loop just ends and the function returns ret, still holding
FC_NORMAL_END from the last SolveForCapacitance() call --
"success," with an output matrix that was never populated for that
run. Nothing distinguishes this from a real convergence.

Fix: populate capRe/capIm with the last computed iteration and
return a new, distinct code (FC_CONVERGENCE_FAILED) instead.