I was afraid of that. Clearly, the contractor was trying to save a few bucks....but now you've got a lot of work...but its doable.
A couple of choices: First, you can apply a fiberglas wall paper over your entire bathroom wall, that you apply by first painting the surface, apply the reinforcing mesh, then put two coats of special paint over that. This should cover those joints, but will be fairly expensive and a lot of work.
Second, and the way I would do it, is as follows. Run in some 2" drywall screws at sharp angles into the stud behind the butt joint (please tell me there's a stud back there!!), spaced out about 1 to 1 1/2" from the butt joint. Sink these down about 1/4" below the surface of the drywall (hint: run them down with your cordless to the surface, then sink them down by hand...otherwise, you'll risk going right through the drywall). Then lightly scribe a cut down the wall, 2" on each side of the but joint, just cutting through the drywall paper. Peel the 2" strips of paper off. Next, scrape off the gypsum to a V shape into the butt joint, using a small Stanley sure-form plane (the little one hand rasp). This will create your tapered V. Don't take off too much...only about 1/8" to 3/16" deep at the butt joint. Then put in a thin layer of compound, then your tape, then your over coat of compound and finish as normal.
Don't try to just put tape over the butt joint and finish with drywall compound. I know commercial guys do this, but it requires a lot of skill to taper out this little mound, and the wall will never look flat, expecially with side light.