handle ngspice notes and Trying in stderr

[volpx]

When parsing the ngspice stderr it could happen it throws only information messages like:
Trying gmin = 1.6548E-12 Note: One successful gmin step
or
Note: Starting dynamic gmin stepping
during an operating point analysis.
In my opinion they should be treated like warnings and not errors since the simulation sill goes to completion and gives the results after.