[Documentation] Improve documentation of the interaction with OLS and ENA APIs

[M-casado]

I would appreciate a more profound documentation regarding how Biovalidator behaves with OLS's and ENA's APIs. Specifically, if the APIs are unavailable, what the error messages would look like.
The other day I noticed OLS was down, but I couldn't test how Biovalidator behaves when trying to reach its service and receiving server errors.

In essence, I would like for the documentation of Biovalidator (perhaps here) to mention whether Biovalidator throws an error (and of what type) if their services are unreachable.