[MM-24753] Improves error message when plugin crashes during slash command (#15334)

Co-authored-by: Ali Farooq <ali.farooq0@pm.me>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

app/plugin_commands_test.go

@@ -286,4 +286,94 @@ func TestPluginCommand(t *testing.T) {
 
 		th.App.RemovePlugin(pluginIds[0])
 	})
+	t.Run("plugin has crashed before execution of command", func(t *testing.T) {
+		tearDown, pluginIds, activationErrors := SetAppEnvironmentWithPlugins(t, []string{`
+			package main
+
+			import (
+				"github.com/mattermost/mattermost-server/v5/plugin"
+				"github.com/mattermost/mattermost-server/v5/model"
+			)
+
+			type MyPlugin struct {
+				plugin.MattermostPlugin
+
+			}
+
+			func (p *MyPlugin) OnActivate() error {
+				err := p.API.RegisterCommand(&model.Command{
+					Trigger: "code",
+				})
+				if err != nil {
+					p.API.LogError("error", "err", err)
+				}
+				panic("Uncaught Error")
+
+				return err
+			}
+
+			func (p *MyPlugin) ExecuteCommand(c *plugin.Context, args *model.CommandArgs) (*model.CommandResponse, *model.AppError) {
+				return &model.CommandResponse{}, nil
+			}
+
+			func main() {
+				plugin.ClientMain(&MyPlugin{})
+			}
+		`}, th.App, th.App.NewPluginAPI)
+		defer tearDown()
+		require.Len(t, activationErrors, 1)
+		require.Nil(t, nil, activationErrors[0])
+		args.Command = "/code"
+		resp, err := th.App.ExecuteCommand(args)
+		require.Nil(t, resp)
+		require.NotNil(t, err)
+		require.Equal(t, err.Id, "model.plugin_command_error.error.app_error")
+		th.App.RemovePlugin(pluginIds[0])
+	})
+
+	t.Run("plugin has crashed due to the execution of the command", func(t *testing.T) {
+		tearDown, pluginIds, activationErrors := SetAppEnvironmentWithPlugins(t, []string{`
+			package main
+
+			import (
+				"github.com/mattermost/mattermost-server/v5/plugin"
+				"github.com/mattermost/mattermost-server/v5/model"
+			)
+
+			type MyPlugin struct {
+				plugin.MattermostPlugin
+
+			}
+
+			func (p *MyPlugin) OnActivate() error {
+				err := p.API.RegisterCommand(&model.Command{
+					Trigger: "code",
+				})
+				if err != nil {
+					p.API.LogError("error", "err", err)
+				}
+
+				return err
+			}
+
+			func (p *MyPlugin) ExecuteCommand(c *plugin.Context, args *model.CommandArgs) (*model.CommandResponse, *model.AppError) {
+				panic("Uncaught Error")
+				return &model.CommandResponse{}, nil
+			}
+
+			func main() {
+				plugin.ClientMain(&MyPlugin{})
+			}
+		`}, th.App, th.App.NewPluginAPI)
+		defer tearDown()
+		require.Len(t, activationErrors, 1)
+		require.Nil(t, nil, activationErrors[0])
+		args.Command = "/code"
+		resp, err := th.App.ExecuteCommand(args)
+		require.Nil(t, resp)
+		require.NotNil(t, err)
+		require.Equal(t, err.Id, "model.plugin_command_crash.error.app_error")
+		th.App.RemovePlugin(pluginIds[0])
+	})
+
 }