Do not store error if context/occurrence is not valid

[odarriba]

This change solves an issue in which an invalida content of the error context case the Error schema to be stored but not the Occurrence, which means that the error appears on the dashboard list but cannot be opened because the details were not stored.

On this change I change the store procedure to use a DB transaction and ensure that, if something fails, no data is stored and an exception is generated.

I thought about storing that exception ourselves but if the issue is not related to the data stored but other parts of the stack (network, DB, etc) it can end up in an infinite loop.

I have also updated the documentation so it states the requirements needed for the context data to be stored (basically, it has to be convertible to JSON).

Closes #65

[jaimeiniesta]

Hey, that's nice, I just tried to put the whole User struct and I got this, as expected. Good work, thanks!