bitbake: fetch2: avoid printing "no checksum" error message twice

bitbake: fetch2: avoid printing "no checksum" error message twice

Because of the way we were handling this error, it was printed twice -
once via logger.error() (to avoid the log being printed) and a second
time when the exception gets wrapped in a FuncFailed at a higher level.
Call logger.error() earlier and change the text we send in the
exception to be more brief, so it more closely resembles the behaviour
when there is an invalid checksum.

(Bitbake rev: 46765369d7f76ec7f67b90430131a79eb6a66235)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>