When opening the OS image fails we used to print a message *which* file
could not be opened. This message is no longer printed since we now fail
in read_file_2 when there is an error. Move the message in the
read_file_2 failure path to print it again. file_detect_type() needs no
handling for ret < 0, in case of failure it returns filetype_unknown
which is handled later, so we can remove the error handling there.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>