In plotcapzone, trace line should not return 'no message' or that should be documented

[mbakker7]

In the following example, some trace lines return 'no message'. I think that occurs when they leave the aquifer through the semi-confining layer, but I am not sure. There should be a more informative message.

ml = tfs.ModelMaq(kaq=20, z=[16, 15, 0], c=[400], hstar=30, topboundary='semi')
w1 = tfs.Well(model=ml, xw=200, yw=50, Qw=400, rw=0.2)
w2 = tfs.Well(model=ml, xw=-100, yw=-50, Qw=200, rw=0.2)
ml.solve()
ax = ml.plots.topview(win=[-400, 600, -400, 400])
ml.plots.plotcapzone(w1, tmax=20 * 365, color='C1', nt=20, hstepmax=5, silent=False, 
                     ax=ax)