Fix broken SNI

Summary: There were a bunch of bugs in D1462765, which revealed an older bug t4917174.  Fixes:

1) Use the correct path to the file for stat(2)
2) Don't skip trying to load the cert when stat fails
3) log the errno of stat failure
4) correctly set dup to be true when insert fails
5) re-set the SNI no-match handler every time a cert is added, otherwise it gets cleared and disables SNI handling

Reviewed By: @bertmaher

Differential Revision: D1499536