Added central execution of build commands and basic error handling.

	modified:   buildmanager.drush.inc
Fixed bug. That drush_exec() should have been removed in previous commit.

	modified:   buildmanager.drush.inc
Added improved error reporting when commands fail.
 	modified:   buildmanager.drush.inc
Fixed broken default for buildmanager-configure.

 	modified:   buildmanager.drush.inc