Use singular form of panic and remove the unnecessary
'however', when comparing Goexit's behavior to 'a panic'
as well as what happens for deferred recovers with Goexit.

Change-Id: I3116df3336fa135198f6a39cf93dbb88a0e2f46e
Reviewed-on: https://go-review.googlesource.com/79755Reviewed-by: Rob Pike <r@golang.org>