catch robustness errors from RTriangle

[mdsumner]

Catch the error (or just document) where

DEL0(cst10[c(131, 363), ])
Internal error in segmentintersection():
  Topological inconsistency after splitting a segment.
...

and provide suggestions along the lines of https://github.com/hypertidy/anglr/issues/7#issuecomment-628934182