According to Wikipedia,
Moral luck describes circumstances whereby a moral agent is assigned
moral blame or praise for an action or its consequences even though it
is clear that said agent did not have full control over either the
action or its consequences.
More simply, it is the idea that people are to be held responsible for an action even when they aren't the only force that caused it - even if it occurred accidentally.
This stands a bit in contrast to the typical sentiment that responsibility correlates with voluntary action. (Meaning, if you choose to do something and do it, you are responsible for it.)
There are some interesting problematic examples in the Wikipedia article you might want to check out.
I don't know enough about Gauguin to speculate as to why he's such a good example of moral luck, but hopefully knowledge of the term will make the book's argument clearer in context.
Moral agent and moral agency are terms in ethics (moral philosophy) used in discussions of responsibility, negligence, judgment, and free will. You will also find it in philosophy of law, psychology, theology, and other fields interested in questions of accountability.
The philosophical sense of agent is of a person or thing which performs an action, and agency is the capacity to take that action. That is, an agent is one who possesses agency.
As Blackwell puts it, moral agency is
Any individual who is capable of formulating or following general moral principles and rules, and who has an autonomous will so that he can decide ultimately what acts he should perform and not perform.
Moral agents can react to the acts of other moral agents. Accordingly they are responsible for their acts and are the subject of blame or praise. Adult human beings are paradigmatic moral agents.
Moral agents are contrasted to moral patients: beings that lack rationality and cannot be held morally accountable for their acts.
Various authors in various fields offer more nuanced definitions:
Moral agents are various described— as entities that are causally responsible for actions (Eshleman, 2004; Heider, 1958), as entities that can earn blame or praise for their actions (Shaver, 1985), as entities that know their actions as right or wrong (Edwards, 1790; H. B. Miller, 1994), or as entities that can intend (Bratman, 1987). These definitions allow moral agency to be ascribed to humans, of course, but also to be attributed in limited ways to groups (e.g. corporations, nations; Knobe & Prinz, 2008) and sometimes even to animals (Shapiro, 2006) or mechanical agents, such as robots or computers (Floridi & Sanders, 2004).
Best Answer
The statement in bold simply means that the poet has a strict sense of what is "right".
The words "imperiously" and "unsparing" means a strong person in terms of vision and decision making when it is used for leaders, famous people etc.