Change the error messages in bch2_inconsistent_error() and
bch2_fatal_error() so we can distinguish them.

Also, prefer bch2_fs_fatal_error() (which also logs an error message) to
bch2_fatal_error(), and change a call to bch2_inconsistent_error() to
bch2_fatal_error() when we can't continue.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>