feat: use specific `Error` type in `__ExecutableBuilder::start`

[nmoutschen]

Is your feature request related to a problem? Please describe.

At the moment, __ExecutableBuilder::start uses anyhow::Result<()> as its return type. This means that a custom binary using the apollo-router as a library cannot infer why Apollo Router fails to start, short of parsing the result manually, and without guarantee that new error types won't emerge in new versions.

Describe the solution you'd like

__ExecutableBuilder::start should return a Result<(), StartError>, with StartError being an enum containing different error type variants. E.g.:

enum StartError {
    InvalidConfig(..),
    InvalidSchema(..),
    SchemaNotFound(..),
    // ...
}

Describe alternatives you've considered

• https://github.com/apollographql/router/issues/3231: related, but would help us infer errors at another level, as this would rely on eventually consistent mechanisms instead of immediate inference.
• Expose the error type in the health check endpoint: this would require Apollo Router to keep the health check endpoint running, even on complete failure - which might break how certain systems work today

Additional context Add any other context or screenshots about the feature request here.

[BrynCooke]

Let's see if we can get a design for the structured error messages and see where we stand.