This reverts commit 4ec449a6.

I meant to not push this one.  Review found that a link error is not
mandated: it should link, but you get undefined rendering if you rely
on a missing stage.

     page 42/55 section 2.11 "Vertex Shaders":

     "If the program object has no vertex shader, or no program object
      is currently in use, the results of vertex shader execution are
      undefined."

(and similar for page 160/173 section 3.9 "Fragment Shaders" for FS,
and page 45/58 section 2.11.2 "Program Objects" for program being 0)

It turns out the commit was broken anyway, because it was missing a
"goto done", so linkstatus got smashed back to true later and the
error just showed up as a warning in the infolog.