Did John Calvin believe there was no salvation outside the visible church? What did John Calvin believe about salvation outside of the church?

He writes in book IV of the Institutes:


  But because it is now our intention to discuss the visible church, let us learn even from the simple title “mother” how useful, indeed how necessary, it is that we should know her. For there is no other way to enter into life unless this mother conceive us in her womb, give us birth, nourish us at her breast, and lastly, unless she keep us under her care and guidance until, putting off mortal flesh, we become like the angels [Matthew 22:30]. Our weakness does not allow us to be dismissed from her school until we have been pupils all our lives. Furthermore, away from her bosom one cannot hope for any forgiveness of sins or any salvation, as Isaiah [Isaiah 37:32] and Joel [Joel 2:32] testify. Ezekiel agrees with them when he declares that those whom God rejects from heavenly life will not be enrolled among God’s people [Ezekiel 13:9]. On the other hand, those who turn to the cultivation of true godliness are said to inscribe their names among the citizens of Jerusalem [cf. Isaiah 56:5; Psalm 87:6]. For this reason, it is said in another psalm: “Remember me, O Jehovah, with favor toward thy people; visit me with salvation: that I may see the well-doing of thy chosen ones, that I may rejoice in the joy of thy nation, that I may be glad with thine inheritance” [Psalm 106:4-5 p.; cf. Psalm 105:4, Vg., etc.]. By these words God’s fatherly favor and the especial witness of spiritual life are limited to his flock, so that it is always disastrous to leave the church.
The text you quoted is certainly clear teaching of John Calvin on the subject. He believed that there was no salvation by remaining outside of the visible church, although it is necessary to distinguish that by not assuming that one could somehow not be saved unless inside a church building or attending a worship service. Rather, any person who professed to be a believer but somehow detached themselves from the visible body of Christ (the visible church) could have no hope of salvation or forgiveness of sins.

This is elaborated on in Institutes of Christian Religion, Book 4, Chapter 1:


  God alone knoweth them that are his. Still he has given marks to
  discern his children.
  
  These marks are the ministry of the word, and administration of the
  sacraments instituted by Christ. The same rule not to be followed in
  judging of individuals and of churches.
  
  We must on no account forsake the Church distinguished by such marks.
  Those who act otherwise are apostates, deserters of the truth and of
  the household of God, deniers of God and Christ, violators of the
  mystical marriage.


Here again it is clearly shown that those who forsake the visible church are apostate etc. 

And later in the chapter, in section 10:


  Whence it follows, that revolt from the Church is denial of God and
  Christ. Wherefore there is the more necessity to beware of a dissent
  so iniquitous; for seeing by it we aim as far as in us lies at the
  destruction of God's truth, we deserve to be crushed by the full
  thunder of his anger. No crime can be imagined more atrocious than
  that of sacrilegiously and perfidiously violating the sacred marriage
  which the only begotten Son of God has condescended to contract with
  us.


However, if the church that one was attending did not show the marks of the true church (the ministry of the word, and the administration of the sacraments instituted by Christ), then it was permissible and indeed preferable to leave such an institution. Having said that, to be in this church-less state is a most lamentable occurrence, and one which should be remedied as soon as possible by joining once again to a true church.