crypto/x509: SystemRootsError style tweaks, document in Verify
crypto/x509: SystemRootsError style tweaks, document in Verify

In retrospect this should've been a variable instead of a type,
but oh well.

LGTM=agl
R=agl
CC=golang-codereviews
https://codereview.appspot.com/129250044

sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh