Error Improvements

[ManevilleF]

Hello,

In arangors there are two error types:

• ArangoError which map the ArangoDB HTTP responses whith code, error num and message
• ClientError which is the global error enum, with a ArangoError variant and a few other variants which are irrelevant to the document operation responses.

I can make a Merge requests changes the error type of ArangoDB operations from ClientError to a ArangoError.

With the From<> implementation for ClientError this won't be breaking changes but it will at least allow more specific error handling.

[fMeow]

This is a way to convert from ArangoError to ClientError.

We can either explictly use ClientError::from(arango_error) or simple use a ? to let rust do the conversion for us.