bootm: Handle errors consistently

A recent bootm fix left the error path incomplete. If CONFIG_TRACE is
set it may still not be a supported command, so cover that with the
unsupported subcommand print.  Once we handle BOOTM_STATE_OS_GO, we can
just move into the error handler itself, no need for a goto there.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Update slightly based on Simon's changes to also cover
CONFIG_TRACE/BOOTM_STATE_FAKE_OS_GO]
Signed-off-by: Tom Rini <trini@ti.com>