QiskitOptimizationError thrown when integer variable

Added a sentence to clarify that a quadratic program with integer variable cannot be converted.

Author: omarshehab

tutorials/optimization/3_minimum_eigen_optimizer.ipynb

@@ -29,7 +29,7 @@
     "Qiskit wraps the translation to an Ising Hamiltonian (in Qiskit Aqua also called `Operator`), the call to an `MinimumEigensolver` as well as the translation of the results back to `OptimizationResult` in the `MinimumEigenOptimizer`.\n",
     "\n",
     "In the following we first illustrate the conversion from a `QuadraticProgram` to an `Operator` and then show how to use the `MinimumEigenOptimizer` with different `MinimumEigensolver` to solve a given `QuadraticProgram`.\n",
-    "The algorithms in Qiskit automatically try to convert a given problem to the supported problem class if possible, for instance, the `MinimumEigenOptimizer` will automatically translate integer variables to binary variables or add a linear equality constraints as a quadratic penalty term to the objective.\n",
+    "The algorithms in Qiskit automatically try to convert a given problem to the supported problem class if possible, for instance, the `MinimumEigenOptimizer` will automatically translate integer variables to binary variables or add a linear equality constraints as a quadratic penalty term to the objective. It should be mentioned that Aqua will through a `QiskitOptimizationError` if conversion of a quadratic program with integer variable is attempted.\n",
     "\n",
     "The circuit depth of `QAOA` potentially has to be increased with the problem size, which might be prohibitive for near-term quantum devices.\n",
     "A possible workaround is Recursive QAOA, as introduced in [2].\n",