populate_sdk_ext: Fix confusing failure message

populate_sdk_ext: Fix confusing failure message

This stack trace is horrible and obscurs the real failure message, avoid that.

The stack trace of python calls that resulted in this exception/failure was:
File: 'exec_python_func() autogenerated', lineno: 2, function: <module>
     0001:
 *** 0002:copy_buildsystem(d)
     0003:
File: 'build/meta/classes/populate_sdk_ext.bbclass', lineno: 446, function: copy_buildsystem
     0442:    sdk_ext_type = d.getVar('SDK_EXT_TYPE')
     0443:    if (sdk_ext_type != 'minimal' or sdk_include_toolchain or derivative) and not sdk_include_nativesdk:
     0444:        # Create the filtered task list used to generate the sstate cache shipped with the SDK
     0445:        tasklistfn = d.getVar('WORKDIR') + '/tasklist.txt'
 *** 0446:        create_filtered_tasklist(d, baseoutpath, tasklistfn, conf_initpath)
     0447:    else:
     0448:        tasklistfn = None
     0449:
     0450:    if os.path.exists(builddir + '/cache/bb_unihashes.dat'):
File: 'build/meta/classes/populate_sdk_ext.bbclass', lineno: 182, function: create_filtered_tasklist
     0178:        # Clean out residue of running bitbake, which check_sstate_task_list()
     0179:        # will effectively do
     0180:        clean_esdk_builddir(d, sdkbasepath)
     0181:    finally:
 *** 0182:        os.replace(sdkbasepath + '/conf/local.conf.bak', sdkbasepath + '/conf/local.conf')
     0183:
     0184:python copy_buildsystem () {
     0185:    import re
     0186:    import shutil
Exception: FileNotFoundError: [Errno 2] No such file or directory: 'tmp/work/qemux86-poky-linux/core-image-sato/1.0-r0/sdk-ext/image//opt/poky/3.0/conf/local.conf.bak' -> 'tmp/work/qemux86-poky-linux/core-image-sato/1.0-r0/sdk-ext/image//opt/poky/3.0/conf/local.conf'

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