Move backend source to server/ subpackage

To clarify the organization.

server/src/domain/error.rs

@@ -0,0 +1,22 @@
+use thiserror::Error;
+
+#[allow(clippy::enum_variant_names)]
+#[derive(Error, Debug)]
+pub enum DomainError {
+    #[error("Authentication error for `{0}`")]
+    AuthenticationError(String),
+    #[error("Database error: `{0}`")]
+    DatabaseError(#[from] sqlx::Error),
+    #[error("Authentication protocol error for `{0}`")]
+    AuthenticationProtocolError(#[from] lldap_auth::opaque::AuthenticationError),
+    #[error("Unknown crypto error: `{0}`")]
+    UnknownCryptoError(#[from] orion::errors::UnknownCryptoError),
+    #[error("Binary serialization error: `{0}`")]
+    BinarySerializationError(#[from] bincode::Error),
+    #[error("Invalid base64: `{0}`")]
+    Base64DecodeError(#[from] base64::DecodeError),
+    #[error("Internal error: `{0}`")]
+    InternalError(String),
+}
+
+pub type Result<T> = std::result::Result<T, DomainError>;