Cleaned up/simplified error-handling:

Cleaned up/simplified error-handling:
  - DistutilsOptionError is now documented as it's actually used, ie.
    to indicate bogus option values (usually user options, eg. from
    the command-line)
  - added DistutilsSetupError to indicate errors that definitely arise
    in the setup script
  - got rid of DistutilsValueError, and changed all usage of it to
    either DistutilsSetupError or ValueError as appropriate
  - simplified a bunch of option get/set methods in Command and
    Distribution classes -- just pass on AttributeError most of
    the time, rather than turning it into something else