nit: missing branch error message

Just tried accidentally to check out a non-existent branch:

[jgarzik@pretzel libata-dev]$ git checkout -f upstream
fatal: Needed a single revision

[jgarzik@pretzel libata-dev]$ ls .git/refs/heads/
adma      chs-support  master    pata-drivers       sil24-original  vsc-mwi
adma-mwi  iomap        ncq       promise-sata-pata  sil-lbt
ALL       iomap-step1  passthru  sil24              upstream-fixes

Better error message would be "hey dummy, branch doesn't exist"

	Jeff