Improve error handling when waveform reconstruction fails and plotting of Mf - af fails

1. In pyRing.py on line 2133 and 2167 this catches some more edge cases where waveform reconstruction fails with an AttributeError.

2. In plots.py on line 2622 the error e was not defined. This replaces it with an error trace, for the case when the creation of an Mf - af plot fails.