Remove workaround for errors in MEX files

Because at some point throwing exceptions from MEX files (with mexErrMsgTxt())
was not working under Windows 64-bit, we had designed a workaround to avoid
using exceptions.

Most MEX files were returning an error code as their first (or sometimes last)
argument, and that code would have to be checked from the MATLAB code.

Since this workaround is no longer needed, this commit removes it. As a
consequence, the interface of many MEX files is modified.

For some background, see https://www.dynare.org/pipermail/dev/2010-September/000895.html