Bitbucket Server importer doesn't handle connection errors gracefully

https://sentry.gitlab.net/gitlab/gitlabcom/issues/595072/

SocketError: getaddrinfo: Name or service not known
  net/http.rb:937:in `initialize'
    TCPSocket.open(conn_address, conn_port, @local_host, @local_port)
  net/http.rb:937:in `open'
    TCPSocket.open(conn_address, conn_port, @local_host, @local_port)
  net/http.rb:937:in `block in connect'
    TCPSocket.open(conn_address, conn_port, @local_host, @local_port)
  timeout.rb:93:in `block in timeout'
    return yield(sec)
  timeout.rb:103:in `timeout'
    bl.call(klass)
...
(147 additional frame(s) were not displayed)

SocketError: Failed to open TCP connection to example:443 (getaddrinfo: Name or service not known)
  net/http.rb:939:in `rescue in block in connect'
    raise e, "Failed to open TCP connection to " +
  net/http.rb:936:in `block in connect'
    begin
  timeout.rb:93:in `block in timeout'
    return yield(sec)
  timeout.rb:103:in `timeout'
    bl.call(klass)
  net/http.rb:935:in `connect'
    s = Timeout.timeout(@open_timeout, Net::OpenTimeout) {
...
(146 additional frame(s) were not displayed)

SocketError: Failed to open TCP connection to example:443 (getaddrinfo: Name or service not known)