cmds/guru: Optimize and Fix error handling definition
cmds/guru: Optimize and Fix error handling definition

- Scope and Output does not use definition

- Add error handling for definitionFallback()

- Fix loader.Config is effective?
  - IgnoreFuncBodies:         true,
  - DisableUnusedImportCheck: true,

Signed-off-by: Koichi Shiraishi <13fbd79c3d390e5d6585a21e11ff5ec1970cff0c@zchee.io>

zchee/nvim-go,zchee/nvim-go