s/Second borrow/Previous borrow/ in error messages.

When a borrow occurs twice illegally, Rust will label the other borrow
as the "second borrow".  This is quite confusing, as the "second borrow"
usually happened before the flagged borrow (e.g. as far as dataflow
is concerned, the first borrow is OK, the second borrow is illegal.)
This patch renames "second borrow" to "previous borrow", to make the
spatial relationship between the two borrows clearer.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>