Change serial "open" functions to throw exception

Change serial "open" functions to throw exception

remote.c assumes that a failure to open the serial connection will set
errno.  This is somewhat true, because the Windows code tries to set
errno appropriately -- but only somewhat, because it isn't clear that
the "pex" code sets it, and the tcp code seems to do the wrong thing.
It seems better to simply have the serial open functions throw on
error.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30770