Merge branch 'rj/pager-die-upon-exec-failure'

Merge branch 'rj/pager-die-upon-exec-failure'

When GIT_PAGER failed to spawn, depending on the code path taken,
we failed immediately (correct) or just spew the payload to the
standard output (incorrect).  The code now always fail immediately
when GIT_PAGER fails.

* rj/pager-die-upon-exec-failure:
  pager: die when paging to non-existing command