Error return instead of assert for external errors.

Error return instead of assert for external errors.

If an error might occur by external reasons,
(invalid input/app behaviors and inappropriate environments)
it should not simply assert away, but should provide
appropriate error messages and handling.

Refactored everything except for tensor_filter_python.cc,
which will be refactored with exception throwing.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>