Better error reporting if unable to find module

Upsilon/Binder/Binder.cs

@@ -248,11 +248,20 @@ namespace Upsilon.Binder
                         {
                             var moduleName = be.Value.ToString();
                             var module = _script.Options.ModuleHandler.GetModule(_script, moduleName);
-                            foreach (var moduleVariable in module.Scope.Variables)
+                            if (module == null)
                             {
-                                if (moduleVariable.Value.Local)
-                                    continue;
-                                Scope.AssignToNearest(moduleVariable.Value);
+                                _diagnostics.LogError(
+                                    $"Can't find module '{moduleName}' in folder '{_script.Options.ScriptLoader.ModulesPath}'",
+                                    parameter.Span);
+                            }
+                            else
+                            {
+                                foreach (var moduleVariable in module.Scope.Variables)
+                                {
+                                    if (moduleVariable.Value.Local)
+                                        continue;
+                                    Scope.AssignToNearest(moduleVariable.Value);
+                                }
                             }
                         }
                     }

Upsilon/ModuleHandler.cs

@@ -14,6 +14,8 @@ namespace Upsilon
                 return module;
 
             var moduleScript = script.Options.ScriptLoader.LoadModule(name);
+            if (moduleScript == null)
+                return null;
             var parsed       = Executor.ContinueWith(script, moduleScript);
             module = parsed.Bind();
             _cachedModules.Add(name, module);

Upsilon/StandardLibraries/BasicFunctions.cs

@@ -47,6 +47,10 @@ namespace Upsilon.StandardLibraries
         {
             var file = fileName.Value;
             var module = script.Options.ModuleHandler.GetModule(script, file);
+            if (module == null)
+            {
+                throw new Exception($"Unable to load module {file} at path {script.Options.ScriptLoader.ModulesPath}");
+            }
             var s = Executor.ContinueWith(script, module);
             var val = s.Evaluate();
             return val != null ? val.ToScriptType() : new ScriptNull();