[entgql] EarlySourceInjector and LateSourceInjector return error

[cliedeman]

What happened?

The EarlySourceInjector and LateSourceInjector plugins do not allow returning an error.
When working on the entgql plugin it would have been useful to be able to return an error here instead of relying on panics.

func (e *EntGQL) InjectSourceEarly() *ast.Source {
	input := e.print()
	if e.debug {
		fmt.Printf("Generated Graphql:\n%s", input)
	}

	return &ast.Source{
		Name:    "entgql.graphql",
		Input:   input,
		BuiltIn: false,
	}
}

You could generating all the source upfront when the plugin is initialized but it makes less readable code.

We do a lot of generation in early source injector because we will need the updated schema for later plugins

[cliedeman]

PR Created #2011