Cosmos Errors

[rylev]

This PR makes several large changes to how error propogation is done in the cosmos crate:

• Make CosmosError an enum with all errors that are possible to get from using a cosmos API. Some of these will be simple wrappers around generic errors from other crates (e.g., serde_json::Error and azure_core::HeaderError)
• Use thiserror for declaring errors. Previously, cosmos used failure which has been deprecated in favor of thiserror.

[ctaggart]

This is looking good. I will have to attempt updating the services to use thiserror.

[thomastaylor312]

One small blocking change and a few optional nits!

[MindFlavor]

I love it 👍 .

I am curious on how you plan to use CosmosError to capture request specific errors. Do you plan to have different error enums each with a CosmosError variant along with the response specific ones?

[rylev]

@MindFlavor which request specific errors do you mean? I believe most requests only error for fairly generic reasons (e.g., unexpected HTTP response status code, bad json, etc.)

[MindFlavor]

Sorry I have been cryptic 👼 ...

I meant, do you mean to expand the CosmosError to capture the meaning of the returned errors? Right now we just return UnexpectedHTTPStatusCode but your work gives us the chance to enhance the user experience by mapping the results with this table: https://docs.microsoft.com/en-us/rest/api/cosmos-db/http-status-codes-for-cosmosdb.

For example, instead of returning "UnexpectedHTTPStatusCode: we expected 200 but we've got 429 back" we could return a variant like CosmosError::TooManyRequests(expected: 200). Also we can mark this type of error as transient.

What do you think?

[rylev]

Closing this for now. I'll reopen with a new PR soon.