Change serial_setbaudrate to throw exception

Change serial_setbaudrate to throw exception

remote.c has this code:

      if (serial_setbaudrate (rs->remote_desc, baud_rate))
	{
	  /* The requested speed could not be set.  Error out to
	     top level after closing remote_desc.  Take care to
	     set remote_desc to NULL to avoid closing remote_desc
	     more than once.  */
	  serial_close (rs->remote_desc);
	  rs->remote_desc = NULL;
	  perror_with_name (name);

The perror here cannot be correct, because if serial_setbaudrate did
set errno, it may be obscured by serial_close.

This patch changes serial_setbaudrate to throw an exception instead.

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