mt_plots_net should throw better error when all edges are filtered out

[KelseyChetnik]

In GitLab by @kelsey.chetnik on Jan 23, 2021, 13:35

When corr_filter is set too low and all edges are removed, the function throws an error that's hard to interpret. The function should catch this case and throw a proper error.

[KelseyChetnik]

In GitLab by @kelsey.chetnik on @krumsiek on Jan 24, 2021, 17:56

Clarification: I mean the case when no edge meets e.g. the p-value cutoff.