If you read the chapter books and the fluff in the codexes some chapters disregard humans as pitiful creatures that need their protections, while others are envious of them knowing that they have sacrificed their lives to allow humans to live in relative obscurity. Then there is the gambit in between. When I fluff up a DIY chapter mine always view themselves as imperfect in the eyes of the emperor and are on an eternal quest to right some slight (perceived as it may be) to once again regain the favor of the emperor and that normal humans hold a higher place in the eyes of the Emperor.
The different chapters each views failure and imperfection in a different way. The Iron Hands view failure as a failure of the flesh and find reason to further augment themselves. Others, particularly chapters formed after the second founding, that follow a more mainstream version of the Imperial Cult (the Emperor is a God, not a Father as it would be for first and second founding chapters) view failure more as a crisis of faith. They were not devout enough or their faith was shaken and that is why they faltered or failed. Creating religious zealots is one of the best ways of indoctrination and coercion. You are not just disappointing your commanding officer or your friends, or you father, you are disappointing God (the Emperor) and that is one of the gravest of sins. First and second founding chapters who revere the Emperor more as a father figure see mistakes and imperfections on a more personal level like you disappointing a father. There are some with bigger chips on their shoulders that view failure more harshly. The Emperor chose them to be the defenders of humanity and failure is simply not an option. Honor and duty are driven into the minds of these marines at indoctrination and if they come from imperial held worlds and not feral worlds faith and loyalty to the Emperor is taught at birth.
There is an entire culture built around the Emperor as the pillar of faith and guidance. The Astartes being designated the defenders of Humanity have little option but to achieve their goals or watch their Emperors domain come to ruination. It is hard to find a corollary to the Marines in modern times, to find an individual so given over to a cause and an ideal that they would fight and die for that goal no matter the cost. The Emperor says take that hill so they take that hill. He says destroy that world so they destroy that world. He says stand here and soak this hail of bullets for me and they gladly give of their lives for the Emperor.
So to answer your question, failure or imperfection is viewed in a varying many ways by the spectrum of chapters, however it is never considered a good thing. The Emperor created the Marines from his own blood and to not live up to that ideal and that legacy is to forsake the Emperor.
The depth of the world that Games Workshop has created through the table top game is expansive to say the least. That said there is also room for other interpretations, later chapters distance themselves further and further from the Emperor and may even claim that he is a propaganda tool of Adeptus Terra to keep the rest of humanity in line and swear fealty only to Adeptus Terra (such chapters may be excommunicated for such heretical views)