IMHO, the best way to fix it is to have someone put on a "skim coat" to bring it back to smooth. Then they can texture it if you want. Sanding it down is a very messy job and would probably cost more. I don't think you can get the proper texture without the skim coat first, but I'm curious to hear other opinions.

There is no easy fix for this that I can think of. If the job is really bad, the contractor may have some legal obligation to make it good. A skim coat, as ventupete suggested, is probably the best option. I'd also suggest you rethink the textured wall idea. They tend to hold dirt, are difficult to clean and repaint and look pretty awful after a year or so.

There might be a problem with adhesion by skim coating over a painted wall surface. For grip, you can get fiberglass mesh in 36" wide rolls. It is similar to the mesh tape for drywall seams only the mesh is a little heavier. You apply it to the wall like wallpaper using spray adhesive. Once that is up and adhered, you can skimcoat as Ventupete suggests. The mesh gives something for the skim coat to grab.