Simpler Error Model for ldk-server

[G8XSU]

Alternative to #19

Main difference being, we will add error_details model later.

Only last 2 commits are of importance.

• Adds proto definition for api errors.
  • When HttpStatusCode is not ok (200), the response content contains a serialized ErrorResponse.
• Add error struct for LdkServerError.
  • It is mainly used as error struct that will be returned from internal layers of ldk-server implementation.
  • It will be converted to proto::error::ErrorResponse at top level service layer.
  • It is intentionally kept flat instead of nesting of fields in enum, so that we can re-use something similar in ldk-server-client. A flat structure without nesting of enums/string might be helpful in case we want to generate ldk-server-client bindings later.

[jkczyz]

LGTM. Please squash.