bitbake: fetch2: improve error output for checksum failures

bitbake: fetch2: improve error output for checksum failures

* Don't print the full exception in the initial warning - if we later
  succeed in fetching the file from a mirror, we won't usually need the
  details (which are in the fetch log if they are needed); otherwise the
  full error will be printed when the fetch operation fails. Also adjust
  the conditional block so that we don't print another warning just
  mentioning we're going to try mirrors.
* Call logger.error() so that with knotty the full log is not printed
* Provide an explanation around the lines we print for easily updating
  the checksums in the recipe. We don't want users to be just blindly
  updating the recipe in case of a transient failure or deliberately
  altered remote file.

(Bitbake rev: 2793413106c925b06783beb7413aa87cbcf246c3)

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