x/net/proxy: SOCKS5 errors are all *errors.errorString

[ger5]

The errors returned by proxy.SOCKS5() are all created with errors.New, meaning package users have to parse strings to determine what happened. At the minimum, they should satisfy net.Error so users can distinguish between timeouts and temporary errors.

[gopherbot]

CL https://golang.org/cl/38278 mentions this issue.

[gopherbot]

Change https://golang.org/cl/41031 mentions this issue: net/http: replace SOCKS client implementation