befs: make consistent use of befs_error()

befs_error() is used in potential errors that could happen in befs to
provide informational log messages. befs_debug() is silent when
CONFIG_BEFS_DEBUG=no, and very verbose when switched on, which is why it is
used for general debugging but not for errors.

Fix a few cases where the befs debug utility usage isn't following the
expected pattern. To make sure we have consistent information in the logs.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>