nats: simplify error check in Conn.connect
nats: simplify error check in Conn.connect

This commit replaces the regex used to check for the string literal
'connection refused' with strings.Contains in `Conn.connect`. Using
regexp.Match requires compiling the regex on each call - since the
string being matched is a literal strings.Contains is a simpler and
more performant option here.

apcera/nats,nats-io/go-nats,wallyqs/go-nats,apcera/nats,nats-io/nats,wallyqs/go-nats,nats-io/go-nats,nats-io/nats