Inconsistent Error Handling and Formatting for 404 Errors in `ReadUser`

[bobbyiliev]

Currently, the ReadUser function handles 404 errors inconsistently:

1. The doRequest method returns nil, fmt.Errorf(...) for 404 errors:
2. Since err != nil, we immediately return the error from ReadUser.
3. Later in ReadUser, an API error handler formats errors differently for non-200 status codes, causing inconsistency: https://github.com/MaterializeInc/terraform-provider-materialize/blob/08b526afd8a7c0c44e07953ecd6d6d071220ec34/pkg/frontegg/user.go#L95-L97

Suggested Changes:

• Consolidate error handling to use consistent string formatting across all endpoints.
• Consider using an Error struct that includes the status code for clarity and uniformity.