Fast-fail on connection errors

[bcleenders]

Avoid hanging on connection errors; fail fast by propagating connection errors (not just for TLS).

This also speeds up tests a lot (from >20s to about 3s), since the tests included some connection timeouts.

This expands on #564 ; it does the same for plaintext connections.

Fixes #387

Demo

Start server that expects TLS (to mimick the issue linked above):

$ go run ./internal/testing/cmd/testserver \
    -cert internal/testing/tls/server.crt \
    -key internal/testing/tls/server.key \
    -p 50051

Without fix:

$ grpcurl -plaintext localhost:50051 list
Failed to dial target host "localhost:50051": context deadline exceeded

With fix:

$ ./grpcurl -plaintext localhost:50051 list
Failed to dial target host "localhost:50051": read tcp 127.0.0.1:64984->127.0.0.1:50051: read: connection reset by peer

[dragonsinth]

nice improvement!

[dragonsinth]

FYI: we've been getting test flakes recently--

--- FAIL: TestBrokenTLS_RequireClientCertButNonePresented (0.02s)
    tls_settings_test.go:332: expecting a TLS certificate error, got: read tcp 127.0.0.1:49228->127.0.0.1:42915: use of closed network connection

It's either this PR, or #564 - unsure. Would you mind taking a look @bcleenders ?