resulttool/resultutils: Fix unicode error handling

This error handling didn't work as expected since upon failure it would
inject bytestreams back into the code leading to tracebacks.

Instead, ignore the decode errors. Fixes:

Traceback (most recent call last):
  File "/home/pokybuild/yocto-worker/a-full/build/scripts/resulttool", line 78, in <module>
    sys.exit(main())
  File "/home/pokybuild/yocto-worker/a-full/build/scripts/resulttool", line 72, in main
    ret = args.func(args, logger)
  File "/home/pokybuild/yocto-worker/a-full/build/scripts/lib/resulttool/store.py", line 70, in store
    resultutils.save_resultsdata(results, tempdir, ptestlogs=True)
  File "/home/pokybuild/yocto-worker/a-full/build/scripts/lib/resulttool/resultutils.py", line 178, in save_resultsdata
    f.write(sectionlog)
TypeError: write() argument must be str, not bytes

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>