This fixes crashes on ARM's Mali drivers, where desktop GL isn't
supported, and epoxy didn't notice the error and tried to dlsym() on
libGL.so.1.
An alternative would have been to look at the response from
eglQueryAPI() first, but that would have meant that we still need to
BindAPI in the case that eglQueryAPI returns something like VG or CL,
and so we needed to handle these errors regardless.
Fixes#22