title: GLPK Integer Optimizer prints "PROBLEX HAS NO INTEGER FEASIBLE SOLUTION" but still returns optimal status
body: <p>Help needed with the output of GLPK. 
  Some constraints cannot be met (intentionally) 
  GLPK prints "PROBLEM HAS NO INTEGER FEASIBLE SOLUTION" 
  but still returns 'optimal' as the status of the solution. </p>
  
  <p>I've set all tolerance levels to 0 </p>
  
  <pre><code>glpk.options['feastol']=0
  solvers.options['feastol']=0
  glpk.options['abstol']=0
  solvers.options['abstol']=0        
  glpk.options['reltol']=0
  solvers.options['reltol']=0
  </code></pre>
  
  <p>This is the output</p>
  
  <pre><code>   GLPK Integer Optimizer, v4.43
     10 rows, 5 columns, 19 non-zeros
     5 integer variables, none of which are binary
     Preprocessing...
     1 hidden covering inequaliti(es) were detected
     5 rows, 5 columns, 14 non-zeros
     5 integer variables, all of which are binary
     Scaling...
      A: min|aij| =  1.000e+00  max|aij| =  1.000e+00  ratio =  1.000e+00
     Problem data seem to be well scaled
     Constructing initial basis...
     Size of triangular part = 4
     Solving LP relaxation...
     GLPK Simplex Optimizer, v4.43
     5 rows, 5 columns, 14 non-zeros
           0: obj =   2.000000000e+00  infeas =  1.000e+00 (1)
     *     2: obj =  -2.500000000e+00  infeas =  0.000e+00 (0)
     *     3: obj =  -4.000000000e+00  infeas =  0.000e+00 (0)
     OPTIMAL SOLUTION FOUND
     Integer optimization begins...
     +     3: mip =     not found yet &gt;=              -inf        (1; 0)
     +     3: mip =     not found yet &gt;=     tree is empty        (0; 1)
     PROBLEM HAS NO INTEGER FEASIBLE SOLUTION
  </code></pre>
  
  <p>The wanted behavior is to indicate failure when "PROBLEM HAS NO INTEGER FEASIBLE SOLUTION"</p>
  
  <p>Thanks.</p>