Where do nimErr's comes from?

[drkameleon]

OK, so... first conclusions:

  * this happens only with `--mm:arc` or `--mm:orc`
  * the vast majority of system calls (or at least a very decent % of them) 
seem to be followed by a `nimErr_` check
  * calls to my own proc's seem to be `nimErr` checked as well, by default
  * setting `{.enforceNoRaises.}` for a proc seems to eliminate the `nimErr` 
check after a call to that specific proc