Need better error message if build config is missing

[aaaaalbert]

Currently (c1b90ad), build_component.py issues this error message when it cannot find the config_build.txt build configuration file:

albert$ python build_component.py 
Building into /Users/albert/RUNNABLE
Traceback (most recent call last):
  File "build_component.py", line 389, in <module>
    main()
  File "build_component.py", line 312, in main
    config_file = open("scripts/config_build.txt")
IOError: [Errno 2] No such file or directory: 'scripts/config_build.txt'

This description is apt, but not too helpful in order to rectify the problem (unless, perhaps, you know the internals of the new build system). The error message should explain the problem nicely, and point the user to the relevant piece of the build instructions.