OCW logic improvement with error handling

[h4n0]

Based on the #51 , an improved version of OCW is required, with implementation of the following points:

• [ ] Make use of newly added errors
• [ ] Update the documentation
• [ ] Consider the OCW to TEE migration

[Satoshi-Kusumoto]

I think it's better to separate TEE related issue to a new one. The scope is so different