Display an error message if ngspice DLL is missing

eeschema/sim/ngspice.cpp

@@ -39,7 +39,9 @@ NGSPICE::NGSPICE()
 #else
     m_dll = new wxDynamicLibrary( "libngspice.so" );
 #endif
-    assert( m_dll );
+
+    if( !m_dll || !m_dll->IsLoaded() )
+        throw std::runtime_error( "Missing ngspice shared library" );
 
     // Obtain function pointers
     m_ngSpice_Init = (ngSpice_Init) m_dll->GetSymbol( "ngSpice_Init" );

eeschema/sim/sim_plot_frame.cpp

@@ -120,6 +120,10 @@ void SIM_PLOT_FRAME::StartSimulation()
 
     /// @todo is it necessary to recreate simulator every time?
     m_simulator.reset( SPICE_SIMULATOR::CreateInstance( "ngspice" ) );
+
+    if( !m_simulator )
+        return;
+
     m_simulator->SetReporter( new SIM_THREAD_REPORTER( this ) );
     m_simulator->Init();
     m_simulator->LoadNetlist( formatter.GetString() );

eeschema/sim/spice_simulator.cpp

@@ -24,8 +24,19 @@
 
 #include "ngspice.h"
 
+#include <confirm.h>
+
 SPICE_SIMULATOR* SPICE_SIMULATOR::CreateInstance( const std::string& )
 {
-    return new NGSPICE;
+    try
+    {
+        return new NGSPICE;
+    }
+    catch( std::exception& e )
+    {
+        DisplayError( NULL, e.what() );
+    }
+
+    return NULL;
 }