The error message is a line from Kierkegaard's Sickness Unto Death which is often thought of as being very difficult to understand, some even think that Kierkegaard was using the line to make fun of how hard it is to understand Hegel. It is mirroring the classes scene from Office Space where the printer displays "PC LOAD LETTER".
In Being and Nothingness Sartre described "The Look" or "The Gaze", which is the phenomenological description of being observed by another, which causes a person to be self conscious, ashamed, and aware of themselves as an object in the world.
Kierkegaard was a pretty angsty dude, even for an existentialist.
In The Brothers Karamazov many of the characters (particularly Dmitri) were extremely worried that if there was no God, "anything goes" (meaning there would be no morality at all). In his novel The Idiot, Dostoyevsky credited Ivan Turgenev with "inventing nihilism" (referring to Turgenev's novel Fathers and Sons).
That's just Nietzsche giving a quote for his job interview. I like to think that Nietzsche would quote himself often in ordinary life, to spice things up.
This is referencing Samuel Beckett's famous play Waiting for Godot.
This is mostly referencing Simone de Beauvoir's work in how an existentialist can have concrete relations with others, in works such as The Ethics of Ambiguity.
Heidegger would use things such as walking in a forest and coming upon a clearing as a metaphor for how Dasein (human beings) would come to understand the world.
Kafka wasn't a big fan of bureaucracy, on the whole.