fetch2: fail checksum validation if SRC_URI checksums set to ""

We were checking SRC_URI md5sum/sha256sum values against None here, so
if they were set to "" then no error was produced. Since the value is
still effectively unset in this case, this is not the right behaviour;
just check if the value doesn't evaluate to False instead.

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

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