Don't fail on convergence trial reporting

[Fatsie]

When ngspice tries an alternative DC converge algorithm it reports the
progress on stderr. This patch makes PySpice not error out when the
alternative DC convergence has been reported to be successful.

Example of report on stderr from spice that previously raised an exception:

Note: Starting dynamic gmin stepping
Trying gmin =   1.0000E-03 Note: One successful gmin step
Trying gmin =   1.0000E-04 Note: One successful gmin step
Trying gmin =   1.0000E-05 Note: One successful gmin step
Trying gmin =   1.0000E-06 Note: One successful gmin step
Trying gmin =   1.0000E-07 Note: One successful gmin step
Trying gmin =   1.0000E-08 Note: One successful gmin step
Trying gmin =   1.0000E-09 Note: One successful gmin step
Trying gmin =   1.0000E-10 Note: One successful gmin step
Trying gmin =   1.0000E-11 Note: One successful gmin step
Trying gmin =   1.0000E-12 Note: One successful gmin step
Trying gmin =   1.0000E-12 Note: One successful gmin step
Note: Dynamic gmin stepping completed

I also saw source stepping that ended with Note: ... finished successfully