howler vs boo-boo

howler and boo-boo both are nouns.

Word	Noun	Adjective	Verb	Adverb
howler	Yes	No	No	No
boo-boo	Yes	No	No	No

As nouns, boo-boo is a hypernym of howler; that is, boo-boo is a word with a broader meaning than howler:

Other hypernyms of howler include bloomer, blooper, blunder, boner, botch, bungle, flub, foul-up, fuckup, pratfall.

howler (noun)	boo-boo (noun)
a glaring blunder	an embarrassing mistake
monkey of tropical South American forests having a loud howling cry
a joke that seems extremely funny