add exception types InvalidBuildspec, InvalidBuildspecSchemaType, InvalidBuildspecExecutor that are used to
raise exceptions when parsing buildspec

The BuildspecError exception now accepts optional argument for 'buildspec' argument

update the codebase to catch the appropriate exception during 'buildtest build', 'buildtest buildspec validate', 'buildtest buildspec find'

…or exception handling when this

class is called. In BuildTest, where we invocate BuildspecParser we call method without checking for
executor hence, InvalidBuildspecExecutor won't be raised

… 'test_BuildspecParser_exceptions' will

check for all exceptions and the other method 'test_BuildspecParser' will test the class logic.
Change behavior of 'walk_tree' method to return absolute path to all files

…cExecutor not ValidationError