Reset last cuda error (#2685)

as error is handled here (exception is being thrown), we do not
want cudaGetLastError() to return any errors.

dlib/cuda/cuda_utils.h

@@ -28,7 +28,7 @@ do{
     {                                                                          \
         std::ostringstream sout;                                               \
         sout << "Error while calling " << #call << " in file " << __FILE__ << ":" << __LINE__ << ". ";\
-        sout << "code: " << error << ", reason: " << cudaGetErrorString(error);\
+        sout << "code: " << cudaGetLastError() << ", reason: " << cudaGetErrorString(error);\
         throw dlib::cuda_error(sout.str());                                          \
     }                                                                          \
 }while(false)