We can tell that "fail" has become an adjective because it can be preceded by the quantifying adverb so, as in "I'm so fail." [Cf. I'm so happy (adj.), but *I'm so student (n.)].
Q: As an SAT writing instructor, I am intrigued by your Grammar Myths page, which debunks the rule that "none" is always singular. Since the College Board follows this rule, we have thousands of students learning to write sentences like “None of the chickens is hatched.” What do you think about that?
Fowler's dictum holds for many other prescriptive rules of grammar. In many cases, ignorance is bliss. Your writing will probably be better, because you're not twisting yourself into knots trying to avoid some illusory error, and you won't waste your time thinking about other people's grammar when you should be attending to their meaning.
There is the illusion that we speak the same language, but we really don't.
I've tried looking this up in every grammar guide I can find, but I haven't found the answer. Consider these two sentences: I bought a car to drive myself to work. I bought an alarm clock to wake me in the morning.