Merge branch 'jc/fsck-dropped-errors'

There were some classes of errors that "git fsck" diagnosed to its
standard error that did not cause it to exit with non-zero status.

* jc/fsck-dropped-errors:
  fsck: exit with non-zero when problems are found

diff --cc builtin/fsck.c
Simple merge

diff --cc t/t1450-fsck.sh
Simple merge