Merge branch 'zj/upstream-error-message'

Error message given when @{u} is used for a branch without its
upstream configured have been clatified.

By Zbigniew Jędrzejewski-Szmek
* zj/upstream-error-message:
  i18n: mark @{upstream} error messages for translation
  Be more specific if upstream branch is not tracked
  Provide better message for barnhc_wiht_tpyo@{u}
  Provide branch name in error message when using @{u}
  t1507: add tests to document @{upstream} behaviour