if post scripts fails, CreatorError is raised to catch script errors such as signing...

if post scripts fails, CreatorError is raised to catch script errors such as signing error

  $ cat PLATFORM.ks
  ...
  %post
  # if you want to catch script error, add set command and trap command at the top of script
  set -e
  trap 'echo KS POST ERROR: command \"$BASH_COMMAND\" failed with error code $?' ERR
  # it will cause an error
  /usr/bin/error_command
  %end
  ...

  $ sudo mic cr auto PLATFORM.ks
  ...
  INFO: Running post scripts ...
  INFO: /tmp/ks-postscript-FShzJp: line 5: /usr/bin/error_command: No such file or directory
  INFO: KS POST ERROR: command "/usr/bin/error_command" failed with error code 1
  INFO:
  ERROR: Failed to execute %post script with '/bin/sh'

Change-Id: I1a34a343b8bdd044ca85939e446c43cce783c900
Signed-off-by: Dohyung Kim <dohyung2.kim@samsung.com>